Reseller hosting is a form of web hosting wherein the account owner has the ability to use his/her allotted hard drive space and bandwidth to host websites on behalf of third parties. The reseller purchases the host's services wholesale and then sells them to his customers for a profit. The certain portion of hard drive and bandwidth is allocated to reseller account. In order to achieve this the reseller may rent a dedicated server from a hosting company or resell shared hosting services. If the latter is the case the reseller is simply given the permission to sell a certain amount of disk space and bandwidth to his own customers without renting a server from a web hosting company he signed for a reseller account with.
The typical web hosting reseller might be a web design firm, web developer or systems integrator who offers web hosting as an add-on service. Reseller hosting is also an inexpensive way for web hosting entrepreneurs to start a company. Most reseller hosting plans allow resellers to create their own service plans and choose their own pricing structure. In many cases, resellers are able to establish their own branding via customized control panels and name servers.
Reseller hosting does not require extensive knowledge of the technical aspects of web hosting. Usually, the data center operator is responsible for maintaining network infrastructure and hardware, and the dedicated server owner configures/secures/updates the server. A reseller is responsible for interfacing with his/her own customer base, but any hardware, software and connectivity problems are typically forwarded to the server provider from whom the reseller plan was purchased.
Through point and click "Control Panels" (as listed below), resellers can set up and manage customer accounts via a web interface. In addition, the ModernBill software is popular among resellers, as it automates account creation and billing. Most of the reseller hosting companies offers different reseller hosting plans. For running the web hosting company control panel is must, we can name it as cpwebhosting(control panel webhosting).
วันศุกร์ที่ 27 มิถุนายน พ.ศ. 2551
One-click hosting
One-click hosting generally describes web services that allows internet users to easily upload one or more files from their hard drives onto the one-click host's server free of charge.
Most such services simply return a URL which can be given to other people, who can then fetch the file later on. As of 2005 these sites have drastically increased in popularity, and subsequently, many of the smaller, less efficient sites have failed. Many internet forums exist in order to share such links; this type of file sharing has, to a degree, taken over from P2P filesharing services[1].
The sites make money through advertising or charging for premium services such as increased downloading capacity, removing any wait restrictions the site may have or prolonging how long uploaded files remain on the site. Many sites implement a CAPTCHA to prevent automated downloading.
Most such services simply return a URL which can be given to other people, who can then fetch the file later on. As of 2005 these sites have drastically increased in popularity, and subsequently, many of the smaller, less efficient sites have failed. Many internet forums exist in order to share such links; this type of file sharing has, to a degree, taken over from P2P filesharing services[1].
The sites make money through advertising or charging for premium services such as increased downloading capacity, removing any wait restrictions the site may have or prolonging how long uploaded files remain on the site. Many sites implement a CAPTCHA to prevent automated downloading.
Dedicated hosting service
A dedicated hosting service, dedicated server, or managed hosting service is a type of Internet hosting where the client leases an entire server not shared with anyone. This is more flexible than shared hosting, as organizations have full control over the server(s), including choice of operating system, hardware, etc. Server administration can usually be provided by the hosting company as an add-on service. In some cases a dedicated server can offer less overhead and a larger return on investment. Dedicated servers are most often housed in data centers, similar to colocation facilities, providing redundant power sources and HVAC systems. In contrast to colocation, the server hardware is owned by the provider and in some cases they will provide support for your operating system or applications.
Self-hosting
Self-hosting refers to the use of a computer program as part of the toolchain or operating system that produces new versions of that same program—for example, a compiler that can compile its own source code. Self-hosting software is commonplace on personal computers and larger systems. Other programs that are typically self-hosting include kernels, assemblers, and shells.
If a system is so new that no software has been written for it, then software is developed on another self-hosting system and placed on a storage device that the new system can read. Development continues this way until the new system can reliably host its own development. Development of the Linux operating system, for example, was initially hosted on a Minix system. Writing new software development tools "from the metal" (that is, without using another host system) is rare and in many cases impossible.
Several programming languages are self-hosting, in the sense that a compiler for the language, written in the same language, is available. The first compiler for a new programming language can be written in another language (in rare cases, machine language) or produced using bootstrapping. Self-hosting languages include Lisp, Forth, Pascal, Delphi, C, Modula-2, Oberon, Smalltalk, OCaml, and FreeBASIC.
History
The first self-hosting compiler (excluding assemblers) was written for Lisp by Hart and Levin at MIT in 1962. Because Lisp interpreters existed previously, but no Lisp compilers, they used an original method to compile their compiler. The compiler, like any other Lisp program, could be run in a Lisp interpreter. So they simply ran the compiler in the interpreter, giving it its own source code to compile.[1]
The compiler as it exists on the standard compiler tape is a machine language program that was obtained by having the S-expression definition of the compiler work on itself through the interpreter. (AI Memo 39)[1]
This technique is only possible when an interpreter already exists for the very same language that is to be compiled. It borrows directly from the notion of running a program on itself as input, which is also used in various proofs in theoretical computer science, such as the proof that the halting problem is undecidable.
If a system is so new that no software has been written for it, then software is developed on another self-hosting system and placed on a storage device that the new system can read. Development continues this way until the new system can reliably host its own development. Development of the Linux operating system, for example, was initially hosted on a Minix system. Writing new software development tools "from the metal" (that is, without using another host system) is rare and in many cases impossible.
Several programming languages are self-hosting, in the sense that a compiler for the language, written in the same language, is available. The first compiler for a new programming language can be written in another language (in rare cases, machine language) or produced using bootstrapping. Self-hosting languages include Lisp, Forth, Pascal, Delphi, C, Modula-2, Oberon, Smalltalk, OCaml, and FreeBASIC.
History
The first self-hosting compiler (excluding assemblers) was written for Lisp by Hart and Levin at MIT in 1962. Because Lisp interpreters existed previously, but no Lisp compilers, they used an original method to compile their compiler. The compiler, like any other Lisp program, could be run in a Lisp interpreter. So they simply ran the compiler in the interpreter, giving it its own source code to compile.[1]
The compiler as it exists on the standard compiler tape is a machine language program that was obtained by having the S-expression definition of the compiler work on itself through the interpreter. (AI Memo 39)[1]
This technique is only possible when an interpreter already exists for the very same language that is to be compiled. It borrows directly from the notion of running a program on itself as input, which is also used in various proofs in theoretical computer science, such as the proof that the halting problem is undecidable.
Free web hosting service
A free web hosting service is a web hosting service that is free, usually advertisement-supported and of limited functionality, though not at all times. Free web hosts will usually provide a subdomain (yoursite.example.com) or a directory (www.example.com/~yourname). In contrast, paid web hosts will usually provide a second-level domain along with the hosting (www.yourname.com). Many free hosts do allow use of separately-purchased domains. Rarely, a free host may also operate as a domain name registrar.
Features and limitations
Only a few free web hosts offer basic package for free and enhanced packages (with more features) for a cost. This allows users to try the service for an initial trial (see how it performs compared to other hosts), and then upgrade when (and if) needed.
Free hosting may have the following limitations:
Limitation on the size of each hosted file
Very small bandwidth per month compared to paid hosting
Disabling on hotlinking of files
File type restrictions (for example MP3, MPEG, ZIP etc.)
Compulsory placement of the Webhosts' Banner or Popup ads into all web pages
No provided uptime guarantee
No allowance of custom URLS, such as "http://www.domain.com". It has to be "http://www.provider.com/domain" or "http://domain.provider.com/".
Some free host may provide these extra features:
A web based control panel
Free email accounts for the domain or subdomain hosted
File transfer via FTP
Scripting languages: PHP, ASP, Perl etc.
Relational databases such as MySQL
Scheduled processes, known as cronjobs
Other features such as guestbooks
Forums and community resources not typical of paid hosts
Reward systems which provide extra free products and services
Have no data limitations offering unlimited space
Features and limitations
Only a few free web hosts offer basic package for free and enhanced packages (with more features) for a cost. This allows users to try the service for an initial trial (see how it performs compared to other hosts), and then upgrade when (and if) needed.
Free hosting may have the following limitations:
Limitation on the size of each hosted file
Very small bandwidth per month compared to paid hosting
Disabling on hotlinking of files
File type restrictions (for example MP3, MPEG, ZIP etc.)
Compulsory placement of the Webhosts' Banner or Popup ads into all web pages
No provided uptime guarantee
No allowance of custom URLS, such as "http://www.domain.com". It has to be "http://www.provider.com/domain" or "http://domain.provider.com/".
Some free host may provide these extra features:
A web based control panel
Free email accounts for the domain or subdomain hosted
File transfer via FTP
Scripting languages: PHP, ASP, Perl etc.
Relational databases such as MySQL
Scheduled processes, known as cronjobs
Other features such as guestbooks
Forums and community resources not typical of paid hosts
Reward systems which provide extra free products and services
Have no data limitations offering unlimited space
Data center
A data center is a facility used to house computer systems and associated components, such as telecommunications and storage systems. It generally includes redundant or backup power supplies, redundant data communications connections, environmental controls (e.g., air conditioning, fire suppression), and special security devices.
History
Data centers have their roots in the huge computer rooms of the early ages of the computing industry. Early computer systems were complex to operate and maintain, and required a special environment in which to operate. Many cables were necessary to connect all the components, and methods to accommodate and organize these were devised, such as standard racks to mount equipment, elevated floors, and cable trays (installed overhead or under the elevated floor). Also, old computers required a great deal of power, and had to be cooled to avoid overheating. Security was important – computers were expensive, and were often used for military purposes. Basic design guidelines for controlling access to the computer room were therefore devised.
During the boom of the microcomputer industry, and especially during the 1980s, computers started to be deployed everywhere, in many cases with little or no care about operating requirements. However, as information technology (IT) operations started to grow in complexity, companies grew aware of the need to control IT resources. With the advent of client-server computing, during the decade of 1990, microcomputers (now called "servers") started to find their places on the old computer rooms. The availability of inexpensive networking equipment, coupled with new standards for network cabling, made it possible to use a hierarchical design which put the servers in a specific room inside the company. The use of the term "data center", as applied to specially design computer rooms, started to gain popular recognition about this time.
The boom of data centers came during the dot-com bubble. Companies needed fast Internet connectivity and non-stop operation to deploy systems and establish a presence on the Internet. Installing such equipment was not viable for many smaller companies. Many companies started building very large facilities, called "Internet data centers", or IDCs, which provide businesses with a range of solutions for systems deployment and operation. New technologies and practices were designed to handle the scale and the operational requirements of such large scale operations. These practices eventually migrated towards the private data centers, and were largely adopted because of their practical results.
As of 2007, data center design, construction, and operation is a well-known discipline. Standard documents from accredited professional groups, such as the Telecommunications Industry Association, specify the requirements for data center design. Well-known operational metrics for data center availability can be used to evaluate the business impact of a disruption. There is still a lot of development being done in operation practice, and also in environmentally-friendly data center design.
History
Data centers have their roots in the huge computer rooms of the early ages of the computing industry. Early computer systems were complex to operate and maintain, and required a special environment in which to operate. Many cables were necessary to connect all the components, and methods to accommodate and organize these were devised, such as standard racks to mount equipment, elevated floors, and cable trays (installed overhead or under the elevated floor). Also, old computers required a great deal of power, and had to be cooled to avoid overheating. Security was important – computers were expensive, and were often used for military purposes. Basic design guidelines for controlling access to the computer room were therefore devised.
During the boom of the microcomputer industry, and especially during the 1980s, computers started to be deployed everywhere, in many cases with little or no care about operating requirements. However, as information technology (IT) operations started to grow in complexity, companies grew aware of the need to control IT resources. With the advent of client-server computing, during the decade of 1990, microcomputers (now called "servers") started to find their places on the old computer rooms. The availability of inexpensive networking equipment, coupled with new standards for network cabling, made it possible to use a hierarchical design which put the servers in a specific room inside the company. The use of the term "data center", as applied to specially design computer rooms, started to gain popular recognition about this time.
The boom of data centers came during the dot-com bubble. Companies needed fast Internet connectivity and non-stop operation to deploy systems and establish a presence on the Internet. Installing such equipment was not viable for many smaller companies. Many companies started building very large facilities, called "Internet data centers", or IDCs, which provide businesses with a range of solutions for systems deployment and operation. New technologies and practices were designed to handle the scale and the operational requirements of such large scale operations. These practices eventually migrated towards the private data centers, and were largely adopted because of their practical results.
As of 2007, data center design, construction, and operation is a well-known discipline. Standard documents from accredited professional groups, such as the Telecommunications Industry Association, specify the requirements for data center design. Well-known operational metrics for data center availability can be used to evaluate the business impact of a disruption. There is still a lot of development being done in operation practice, and also in environmentally-friendly data center design.
DNS hosting service
A DNS hosting service is a service that runs Domain Name System servers. Most, but not all, domain name registrars include DNS hosting service with registration. Free DNS hosting services also exist. Almost all DNS hosting services are "shared"; except for the most popular Internet sites, there is no need to dedicate a server to hosting DNS for a single website. Many third-party DNS hosting services provide Dynamic DNS.
DNS hosting service is better when the provider has multiple servers in various geographic locations that minimize latency for clients around the world.
DNS can also be self-hosted by running DNS software on generic Internet hosting service
Free DNS
A number of sites offer free DNS hosting, either for second level domains registered with registrars which do not offer free (or sufficiently flexible) DNS service, or as third level domains (selection.somedomain.com). These services generally also offer Dynamic DNS. In many cases the free services can be upgraded with various premium services.
DNS hosting service is better when the provider has multiple servers in various geographic locations that minimize latency for clients around the world.
DNS can also be self-hosted by running DNS software on generic Internet hosting service
Free DNS
A number of sites offer free DNS hosting, either for second level domains registered with registrars which do not offer free (or sufficiently flexible) DNS service, or as third level domains (selection.somedomain.com). These services generally also offer Dynamic DNS. In many cases the free services can be upgraded with various premium services.
Internet hosting service
An Internet hosting service is a service that runs Internet servers, allowing organizations and individuals to serve content to the Internet. There are various levels of service and various kinds of services offered.
A common kind of hosting is web hosting. Most hosting providers offer a combined variety of services. Web hosting services also offer e-mail hosting service, for example. DNS hosting service is usually bundled with domain name registration.
Web hosting technology has been causing some controversy lately as Web.com claims that it holds patent rights to the hosting technology with its 19 patents. Hostopia, a large wholesale host, recently purchased a license to use that technology from web.com for 10% of retail revenues [1]. Web.com recently sued Go Daddy as well for similar patent infringement [2].
Generic, yet rather powerful, kinds of Internet hosting provide a server where the clients can run anything they want (including web servers and other servers) and have Internet connections with good upstream bandwidth.
A common kind of hosting is web hosting. Most hosting providers offer a combined variety of services. Web hosting services also offer e-mail hosting service, for example. DNS hosting service is usually bundled with domain name registration.
Web hosting technology has been causing some controversy lately as Web.com claims that it holds patent rights to the hosting technology with its 19 patents. Hostopia, a large wholesale host, recently purchased a license to use that technology from web.com for 10% of retail revenues [1]. Web.com recently sued Go Daddy as well for similar patent infringement [2].
Generic, yet rather powerful, kinds of Internet hosting provide a server where the clients can run anything they want (including web servers and other servers) and have Internet connections with good upstream bandwidth.
Hosting reliability and uptime
Hosting uptime refers to the percentage of time the host is accessible via the internet. Many providers state that they aim for a 99.9% uptime, but there may be server restarts and planned (or unplanned) maintenance in any hosting environment.
A common claim from the popular hosting providers is '99% or 99.9% server uptime' but this often refers only to a server being powered on and doesn't account for network downtime. Real downtime can potentially be larger than the percentage guaranteed by the provider. Many providers tie uptime and accessibility into their own service level agreement (SLA). SLAs sometimes include refunds or reduced costs if performance goals are not met.
A common claim from the popular hosting providers is '99% or 99.9% server uptime' but this often refers only to a server being powered on and doesn't account for network downtime. Real downtime can potentially be larger than the percentage guaranteed by the provider. Many providers tie uptime and accessibility into their own service level agreement (SLA). SLAs sometimes include refunds or reduced costs if performance goals are not met.
Service scope
The scope of hosting services varies widely. The most basic is web page and small-scale file hosting, where files can be uploaded via File Transfer Protocol (FTP) or a Web interface. The files are usually delivered to the Web "as is" or with little processing. Many Internet service providers (ISPs) offer this service free to their subscribers. People can also obtain Web page hosting from other, alternative service providers. Personal web site hosting is typically free, advertisement-sponsored, or cheap. Business web site hosting often has a higher expense.
Single page hosting is generally sufficient only for personal web pages. A complex site calls for a more comprehensive package that provides database support and application development platforms (e.g. PHP, Java, Ruby on Rails, and ASP.NET). These facilities allow the customers to write or install scripts for applications like forums and content management. For e-commerce, SSL is also highly recommended.
The host may also provide an interface or control panel for managing the Web server and installing scripts as well as other services like e-mail. Some hosts specialize in certain software or services (e.g. e-commerce). They are commonly used by larger companies to outsource network infrastructure to a hosting company. To find a web hosting company, searchable directories can be used. One must be extremely careful when searching for a new company because many of the people promoting service providers are actually affiliates and the reviews are biased.
Single page hosting is generally sufficient only for personal web pages. A complex site calls for a more comprehensive package that provides database support and application development platforms (e.g. PHP, Java, Ruby on Rails, and ASP.NET). These facilities allow the customers to write or install scripts for applications like forums and content management. For e-commerce, SSL is also highly recommended.
The host may also provide an interface or control panel for managing the Web server and installing scripts as well as other services like e-mail. Some hosts specialize in certain software or services (e.g. e-commerce). They are commonly used by larger companies to outsource network infrastructure to a hosting company. To find a web hosting company, searchable directories can be used. One must be extremely careful when searching for a new company because many of the people promoting service providers are actually affiliates and the reviews are biased.
Web hosting service
Web hosting service
A web hosting service is a type of Internet hosting service that allows individuals and organizations to provide their own website accessible via the World Wide Web. Web hosts are companies that provide space on a server they own for use by their clients as well as providing Internet connectivity, typically in a data center. Web hosts can also provide data center space and connectivity to the Internet for servers they do not own to be located in their data center, called colocation.
วันอาทิตย์ที่ 15 มิถุนายน พ.ศ. 2551
Eternal Sunshine of the Domainer Mind
Eternal Sunshine of the Domainer MindBy : Todd MintzSo, it is lunchtime on 5/22/07 and I'm looking through the news stories at CNN.com when I come across the following headline that catches my eye:"FDA Approves No Period Birth Control Pill"Why would a guy like me be interested in such news? Well, I've focused some of my affiliate marketing efforts in the area of "lifestyle pharmaceuticals" and birth control pills aren't really too far outside of that focus.I also consider myself a "domainer-in-training" and I'm making a conscious effort to vet every article that I read for a possible domain name purchase.Can I pull some valuable domain names from this breaking news? Let me look a bit closer...Lybrel, the trademarked name of the drug, is definitely off limits as a component of a potential name (since I don't want to be sued) so I have to determine what combination of descriptive words people might type into their browser as a dot com domain if they are searching for information about Lybrel.A quick perusal of the news stories on the topic gave me the one obvious answer:"No Period Birth Control"Then, I went to my domain registrar and, lucky for me, the following domain names were available.* Noperiodbirthcontrol.com* Noperiodbirthcontrolpill.com* Noperiodbirthcontrolpills.comSo, I purchased them. As of this moment, Google is showing 55,700 results for "No Period Birth Control" (quotes included) and I suspect that number is going to grow much higher very quickly...after all, the product isn't even on the market yet.I feel I had a profitable lunch.About the AuthorTodd Mintz is the Director of Internet Marketing & Information Systems for S.R. Clarke Inc., a Real Estate Development and Residential / Commercial Construction Executive Search / Recruiting Firm headquartered in Fairfax, VA with offices nationwide. He is also a Director & Founding Member of SEMpdx: Portland, Oregon's Search Engine Marketing Association.
Claiming An Infringement On Your Domain Name
Claiming An Infringement On Your Domain NameBy : Richard A. ChapoA domain name is your address on the web, which can make it vitally important. So, what do you do when someone tries to move onto your yard?If you work on the web at all, you know that domain names can be very similar. This raises the question of when one domain crosses the line and infringes on another. The issue is often brought up with larger commercial sites when others register similar domains.A claim for domain name infringement is governed by the Uniform Domain Name Dispute Resolution Policy issued by ICANN. As a domain owner, you might be surprised to learn you have consented to this. It occurred when you purchased your domain, although you probably didn't read the terms and conditions associated with your purchase.To prove your domain is being infringed upon, you have to prove certain things. They are as follows:1. You must prove the domain name in dispute is identical or confusingly similar to a trade or service mark you have registered. In simple terms, this means you must show the other guy is using a domain that most people would associated with your business. If I have a trademark for ZipIt and someone is using a NET suffix of this, it is confusing for the public.2. You must also prove the owner of the other domain has no rights or legitimate interests in the domain.3. Finally, you must prove domain name is registered by the other party and being used in bad faith. Indications of bad faith include the other party tried to sell it to you or one of your competitors for more than they paid for it, the other party has done this repeatedly to other companies, the other party registered the domain in an attempt to intentionally steal your clients.If you decide to pursue an infringement action, it is always handled as a binding arbitration. This essentially means the dispute is heard by a panel of arbitrators and their decision is binding upon you and the other party. The advantage of this is it tends to be less expensive than going to court, but your damages are limited to an act instead of money. Specifically, the panel can only issue instructions for the offending domain to be canceled, modified or left alone. The panel cannot award any monetary damages.At the end of the day, using the infringement arbitration provisions of the Uniform Domain Name Dispute Resolution Policy provided by ICANN is a good choice if you just want to terminate a domain you feel is infringing upon yours. The key to the dispute, however, is the fact you need to have a trademark or service mark first in most cases.About the AuthorProtect yourself with a domain name purchase agreement from SanDiegoBusinessLawFirm.com.
Premium domain names
In the business of marketing domain names, "premium" domain names are often valuable, and have particular characterizations: The names are short and memorable, may contain words that are regularly searched on search engines or keywords that help the name gain a higher ranking on search engines. They may contain generic words, so the word has more than one meaning, and they may contain common typos.
Unconventional domain names
Unconventional domain names
Due to the rarity of one-word dot-com domain names, many unconventional domain names, domain hacks, have been gaining popularity. They make use of the top-level domain as an integral part of the Web site's title. Two popular domain hack Web sites are del.icio.us and blo.gs, which spell out "delicious" and "blogs", respectively.
Unconventional domain names are also used to create unconventional email addresses. Non-working examples that spell 'James' are j@m.es and j@mes.com, which use the domain names m.es (of Spain's .es) and mes.com, respectively.
Due to the rarity of one-word dot-com domain names, many unconventional domain names, domain hacks, have been gaining popularity. They make use of the top-level domain as an integral part of the Web site's title. Two popular domain hack Web sites are del.icio.us and blo.gs, which spell out "delicious" and "blogs", respectively.
Unconventional domain names are also used to create unconventional email addresses. Non-working examples that spell 'James' are j@m.es and j@mes.com, which use the domain names m.es (of Spain's .es) and mes.com, respectively.
Subdomain
For a protein domain inside of a larger domain, see Protein domains.
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain.[1] For example, "example.com" is a subdomain of the "com" top-level domain (TLD) while "www.example.com" is a subdomain in the domain "example.com". In fact, the "com" TLD is a subdomain of the root domain, ".". This hierarchical organisation is similar to that in a filesystem; something is a subdomain if it could be equated to a folder, and a record within that subdomain to a file. Note, though, that DNS names are written in descending hierarchy right-to-left, where filesystems are written left-to-right.
Relative to a subdomain, the larger domain that it is a part of is its parent domain, or alternately superdomain (the former term appears to be preferred by the IETF).
Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
A subdomain is sometimes termed a vanity domain, especially when it is a subdomain of an ISP's own domain aliased to an individual user account. However, the term "vanity domain" has other usages, discussed at that article.
Some websites use a different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2, etc.
Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.
Depending on application, a record inside a domain, or subdomain might refer to a Host name, or a service provided by a number of machines in a cluster.
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain.[1] For example, "example.com" is a subdomain of the "com" top-level domain (TLD) while "www.example.com" is a subdomain in the domain "example.com". In fact, the "com" TLD is a subdomain of the root domain, ".". This hierarchical organisation is similar to that in a filesystem; something is a subdomain if it could be equated to a folder, and a record within that subdomain to a file. Note, though, that DNS names are written in descending hierarchy right-to-left, where filesystems are written left-to-right.
Relative to a subdomain, the larger domain that it is a part of is its parent domain, or alternately superdomain (the former term appears to be preferred by the IETF).
Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
A subdomain is sometimes termed a vanity domain, especially when it is a subdomain of an ISP's own domain aliased to an individual user account. However, the term "vanity domain" has other usages, discussed at that article.
Some websites use a different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2, etc.
Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.
Depending on application, a record inside a domain, or subdomain might refer to a Host name, or a service provided by a number of machines in a cluster.
Top-level domain
"TLD" redirects here. For other uses, see TLD (disambiguation).
A top-level domain (TLD), sometimes referred to as a top-level domain name (TLDN), is the last part of an Internet domain name; that is, the letters which follow the final dot of any domain name. For example, in the domain name www.example.com, the top-level domain is com (or COM, as domain names are not case-sensitive). Management of top-level domains is handled by the ICANN.
The Internet Assigned Numbers Authority (IANA) currently classifies top-level domains into three types:
country code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long, for example .us for the United States. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
generic top-level domains (gTLD): Used (at least in theory) by a particular class of organizations (for example, .com for commercial organizations). It is three or more letters long. Most gTLDs are available for use worldwide, but for historical reasons .mil (military) and .gov (governmental) are restricted to use by the respective U.S. authorities. gTLDs are subclassified into sponsored top-level domains (sTLD), e.g. .aero, .coop and .museum, and unsponsored top-level domains (uTLD), e.g. .biz, .info, and .name.
infrastructure top-level domains (iTLD): The top-level domain .arpa is the only confirmed one. .root has been known to exist without reason.
A full list of currently existing TLDs can be found at the list of Internet top-level domains.
Historical TLDs
A .nato was added in the late 1980s by the NIC for the use of NATO, who felt that none of the then existing TLDs adequately reflected their status as an international organization. Soon after this addition, however, the NIC created the .int TLD for the use of international organizations, and persuaded NATO to use nato.int instead. However, the nato TLD, although no longer used, was not deleted until July 1996.
Other historical TLDs are .cs for Czechoslovakia (now .cz for Czech Republic and .sk for Slovak Republic), and .zr for Zaire (now .cd for Democratic Republic of the Congo). In contrast to these, the TLD .su has remained in active use despite the demise of the Soviet Union that it represents, though .ru is most commonly used for Russian domains.
Commercial use of country code TLDs
A number of the world's smallest countries have licensed their TLDs for world-wide commercial use. For example, Niue, a tiny island in the South Pacific Ocean, has licensed the .nu TLD and it is used various places around the world, for example in Denmark, the Netherlands and Sweden, where the word "nu" means "now". Similarly, Tuvalu and the Federated States of Micronesia, other small islands in the South Pacific, have partnered with VeriSign and FSM Telecommunications respectively, to sell domain names using the .tv and .fm TLDs to television and radio stations. Cocos (Keeling) Islands of Australia promoted the use of their .cc as "the next .com", which garnered popularity due to its relative cheapness compared to .com registration at the time.
Pseudo-domains
In the past the Internet was just one of many wide-area computer networks. Computers not connected to the Internet, but connected to another network such as BITNET, CSNET or UUCP, could generally exchange e-mail with the Internet via e-mail gateways. When used on the Internet, addresses on these networks were often placed under pseudo-domains such as .bitnet, .oz, .csnet, and .uucp; however these pseudo-domains implemented in mail server configurations such as sendmail.cf and were not real top-level domains and did not exist in DNS.
Most of these networks have long since ceased to exist, and although UUCP still gets significant use in parts of the world where Internet infrastructure has not yet become well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains now largely survive as historical relics. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the .swift pseudo-domain.[1]
The anonymity network Tor has a pseudo-domain onion, which can only be reached with a Tor client because it uses the Tor-protocol (onion routing) to reach the hidden service in order to protect the anonymity of the domain.
.local deserves special mention as it is required by the Zeroconf protocol. It is also used by many organizations internally, which will become a problem for those users as Zeroconf becomes more popular. Both .site and .internal have been suggested for private usage, but no consensus has yet emerged[citation needed].
A top-level domain (TLD), sometimes referred to as a top-level domain name (TLDN), is the last part of an Internet domain name; that is, the letters which follow the final dot of any domain name. For example, in the domain name www.example.com, the top-level domain is com (or COM, as domain names are not case-sensitive). Management of top-level domains is handled by the ICANN.
The Internet Assigned Numbers Authority (IANA) currently classifies top-level domains into three types:
country code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long, for example .us for the United States. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
generic top-level domains (gTLD): Used (at least in theory) by a particular class of organizations (for example, .com for commercial organizations). It is three or more letters long. Most gTLDs are available for use worldwide, but for historical reasons .mil (military) and .gov (governmental) are restricted to use by the respective U.S. authorities. gTLDs are subclassified into sponsored top-level domains (sTLD), e.g. .aero, .coop and .museum, and unsponsored top-level domains (uTLD), e.g. .biz, .info, and .name.
infrastructure top-level domains (iTLD): The top-level domain .arpa is the only confirmed one. .root has been known to exist without reason.
A full list of currently existing TLDs can be found at the list of Internet top-level domains.
Historical TLDs
A .nato was added in the late 1980s by the NIC for the use of NATO, who felt that none of the then existing TLDs adequately reflected their status as an international organization. Soon after this addition, however, the NIC created the .int TLD for the use of international organizations, and persuaded NATO to use nato.int instead. However, the nato TLD, although no longer used, was not deleted until July 1996.
Other historical TLDs are .cs for Czechoslovakia (now .cz for Czech Republic and .sk for Slovak Republic), and .zr for Zaire (now .cd for Democratic Republic of the Congo). In contrast to these, the TLD .su has remained in active use despite the demise of the Soviet Union that it represents, though .ru is most commonly used for Russian domains.
Commercial use of country code TLDs
A number of the world's smallest countries have licensed their TLDs for world-wide commercial use. For example, Niue, a tiny island in the South Pacific Ocean, has licensed the .nu TLD and it is used various places around the world, for example in Denmark, the Netherlands and Sweden, where the word "nu" means "now". Similarly, Tuvalu and the Federated States of Micronesia, other small islands in the South Pacific, have partnered with VeriSign and FSM Telecommunications respectively, to sell domain names using the .tv and .fm TLDs to television and radio stations. Cocos (Keeling) Islands of Australia promoted the use of their .cc as "the next .com", which garnered popularity due to its relative cheapness compared to .com registration at the time.
Pseudo-domains
In the past the Internet was just one of many wide-area computer networks. Computers not connected to the Internet, but connected to another network such as BITNET, CSNET or UUCP, could generally exchange e-mail with the Internet via e-mail gateways. When used on the Internet, addresses on these networks were often placed under pseudo-domains such as .bitnet, .oz, .csnet, and .uucp; however these pseudo-domains implemented in mail server configurations such as sendmail.cf and were not real top-level domains and did not exist in DNS.
Most of these networks have long since ceased to exist, and although UUCP still gets significant use in parts of the world where Internet infrastructure has not yet become well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains now largely survive as historical relics. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the .swift pseudo-domain.[1]
The anonymity network Tor has a pseudo-domain onion, which can only be reached with a Tor client because it uses the Tor-protocol (onion routing) to reach the hidden service in order to protect the anonymity of the domain.
.local deserves special mention as it is required by the Zeroconf protocol. It is also used by many organizations internally, which will become a problem for those users as Zeroconf becomes more popular. Both .site and .internal have been suggested for private usage, but no consensus has yet emerged[citation needed].
Domain Name System
The Domain Name System (DNS) associates various information with domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. www.example.com, into IP addresses, e.g. 208.77.188.166, which networking equipment needs to deliver information. It also stores other information such as the list of mail servers that accept email for a given domain. In providing a worldwide keyword-based redirection service, the Domain Name System is an essential component of contemporary Internet use.
Uses
The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses.
Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent) independent of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.
The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative name server for each domain to keep track of its own changes, avoiding the need for a central register to be continually consulted and updated.
History
The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era. Back then however, a different system was used, as DNS was invented only in 1983, shortly after TCP/IP was deployed. With the older system, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International)[1][2]. The HOSTS.TXT file mapped numerical addresses to names. A hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address (eg. 208.77.188.166) to use for a hostname (eg. www.example.net) without checking DNS. Systems based on a hosts file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its hosts file.
The growth of networking called for a more scalable system, one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses.
At the request of Jon Postel, Paul Mockapetris invented the Domain Name system in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and RFC 883. In November 1987, the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more-recent RFCs have proposed various extensions to the core DNS protocols.
In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon). Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s.
Due to BIND's long history of security issues and exploits, several alternative nameserver and resolver programs have been written and distributed in recent years.
How DNS works in theory
Domain names, arranged in a tree, cut into zones, each served by a nameserver.
The domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
When a system administrator wants to let another administrator control a part of the domain name space within the first administrator’s zone of authority, control can be delegated to the second administrator. This splits off a part of the old zone into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for the new zone.
Parts of a domain name
A domain name usually consists of two or more parts (technically a label), which is conventionally written separated by dots, such as example.com.
The rightmost label conveys the top-level domain (for example, the address www.example.com has the top-level domain com).
Each label to the left specifies a subdivision, or subdomain of the domain above it. Note: “subdomain” expresses relative dependence, not absolute dependence. For example: example.com comprises a subdomain of the com domain, and www.example.com comprises a subdomain of the domain example.com. In theory, this subdivision can go down 127 levels. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 253 characters. [3] In practice, some domain registries may have shorter limits.
A hostname refers to a domain name that has one or more associated IP addresses; ie: the 'www.example.com' and 'example.com' domains are both hostnames, however, the 'com' domain is not.
DNS servers
Main article: Name server
The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).
DNS resolvers
See also: resolv.conf
A resolver looks up the resource record information associated with nodes. A resolver knows how to communicate with name servers by sending DNS queries and heeding DNS responses.
A DNS query may be either a recursive query or a non-recursive query:
A non-recursive query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries.
The resolver (or another DNS server acting recursively on behalf of the resolver) negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function simplistically and can communicate only with a single name server. These simple resolvers rely on a recursive query to a recursive name server to perform the work of finding information for them.
Address resolution mechanism
(This description deliberately uses the fictional .example TLD in accordance with the DNS guidelines themselves.)
In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).
For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult.
A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.
As originally envisaged, the process was as simple as:
the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time.
query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain).
querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.
The diagram illustrates this process for the real host www.wikipedia.org.
The mechanism in this simple form has a difficulty: it places a huge operating burden on the root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. The section DNS in practice describes how this is addressed.
Circular dependencies and glue records
Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.
For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative name server for these lives at ns1.something.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org. Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.something.en.wikipedia.org is located.
In practice
When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world."
Caching and time to live
Because of the huge volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e. the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer. How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks.
Caching time
As a noteworthy consequence of this distributed and caching architecture, changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00noon will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL.
Note that the term "propagation", although very widely used in this context, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain, except your NS records and any authoritative DNS servers that use your domain name).
Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the Start of Authority (SOA) record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).
In the real world
DNS resolving from program to OS-resolver to ISP-resolver to greater system.
Users generally do not communicate directly with a DNS resolver. Instead DNS-resolution takes place transparently in client-applications such as web-browsers, mail-clients, and other Internet applications. When an application makes a request which requires a DNS lookup, such programs send a resolution request to the local DNS resolver in the local operating system, which in turn handles the communications required.
The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. In any event, the name server thus queried will follow the process outlined above, until it either successfully finds a result or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request.
Broken resolvers
An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond.[citation needed]
As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times — on the order of one minute. Internet Explorer offers a notable exception: recent versions cache DNS records for half an hour.[4]
Other applications
The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:
Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.
There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping.
Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.
To provide resilience in the event of computer failure, multiple DNS servers are usually provided for coverage of each domain, and at the top level, thirteen very powerful root servers exist, with additional "copies" of several of them distributed worldwide via Anycast.
Uses
The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses.
Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent) independent of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.
The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative name server for each domain to keep track of its own changes, avoiding the need for a central register to be continually consulted and updated.
History
The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era. Back then however, a different system was used, as DNS was invented only in 1983, shortly after TCP/IP was deployed. With the older system, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International)[1][2]. The HOSTS.TXT file mapped numerical addresses to names. A hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address (eg. 208.77.188.166) to use for a hostname (eg. www.example.net) without checking DNS. Systems based on a hosts file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its hosts file.
The growth of networking called for a more scalable system, one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses.
At the request of Jon Postel, Paul Mockapetris invented the Domain Name system in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and RFC 883. In November 1987, the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more-recent RFCs have proposed various extensions to the core DNS protocols.
In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon). Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s.
Due to BIND's long history of security issues and exploits, several alternative nameserver and resolver programs have been written and distributed in recent years.
How DNS works in theory
Domain names, arranged in a tree, cut into zones, each served by a nameserver.
The domain name space
The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
When a system administrator wants to let another administrator control a part of the domain name space within the first administrator’s zone of authority, control can be delegated to the second administrator. This splits off a part of the old zone into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for the new zone.
Parts of a domain name
A domain name usually consists of two or more parts (technically a label), which is conventionally written separated by dots, such as example.com.
The rightmost label conveys the top-level domain (for example, the address www.example.com has the top-level domain com).
Each label to the left specifies a subdivision, or subdomain of the domain above it. Note: “subdomain” expresses relative dependence, not absolute dependence. For example: example.com comprises a subdomain of the com domain, and www.example.com comprises a subdomain of the domain example.com. In theory, this subdivision can go down 127 levels. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 253 characters. [3] In practice, some domain registries may have shorter limits.
A hostname refers to a domain name that has one or more associated IP addresses; ie: the 'www.example.com' and 'example.com' domains are both hostnames, however, the 'com' domain is not.
DNS servers
Main article: Name server
The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).
DNS resolvers
See also: resolv.conf
A resolver looks up the resource record information associated with nodes. A resolver knows how to communicate with name servers by sending DNS queries and heeding DNS responses.
A DNS query may be either a recursive query or a non-recursive query:
A non-recursive query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries.
The resolver (or another DNS server acting recursively on behalf of the resolver) negotiates use of recursive service using bits in the query headers.
Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function simplistically and can communicate only with a single name server. These simple resolvers rely on a recursive query to a recursive name server to perform the work of finding information for them.
Address resolution mechanism
(This description deliberately uses the fictional .example TLD in accordance with the DNS guidelines themselves.)
In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).
For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult.
A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.
As originally envisaged, the process was as simple as:
the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time.
query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain).
querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.
The diagram illustrates this process for the real host www.wikipedia.org.
The mechanism in this simple form has a difficulty: it places a huge operating burden on the root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. The section DNS in practice describes how this is addressed.
Circular dependencies and glue records
Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.
For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative name server for these lives at ns1.something.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org. Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.something.en.wikipedia.org is located.
In practice
When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world."
Caching and time to live
Because of the huge volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e. the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer. How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks.
Caching time
As a noteworthy consequence of this distributed and caching architecture, changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00noon will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL.
Note that the term "propagation", although very widely used in this context, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain, except your NS records and any authoritative DNS servers that use your domain name).
Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the Start of Authority (SOA) record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308
Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).
In the real world
DNS resolving from program to OS-resolver to ISP-resolver to greater system.
Users generally do not communicate directly with a DNS resolver. Instead DNS-resolution takes place transparently in client-applications such as web-browsers, mail-clients, and other Internet applications. When an application makes a request which requires a DNS lookup, such programs send a resolution request to the local DNS resolver in the local operating system, which in turn handles the communications required.
The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. In any event, the name server thus queried will follow the process outlined above, until it either successfully finds a result or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request.
Broken resolvers
An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond.[citation needed]
As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times — on the order of one minute. Internet Explorer offers a notable exception: recent versions cache DNS records for half an hour.[4]
Other applications
The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:
Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.
There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping.
Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.
To provide resilience in the event of computer failure, multiple DNS servers are usually provided for coverage of each domain, and at the top level, thirteen very powerful root servers exist, with additional "copies" of several of them distributed worldwide via Anycast.
Domain name registrar
A domain name registrar is a company accredited by the Internet Corporation for Assigned Names and Numbers (ICANN) and/or by a national ccTLD authority to register Internet domain names. These "retail" companies are often distinct from the "wholesale" domain name registry operator.
The Internet Corporation for Assigned Names and Numbers, or ICANN, has authority over generic top-level domains, or gTLDs. Examples of gTLDs include .com, .net, .org and .mobi. ICANN does not have authority over ccTLDs, or Country Code Top-Level Domains, though it is quite common for domain name registrars to offer ccTLD registration services as well. Most registrars provide DNS hosting service, but this is not required, and is often considered a separate service.
Until 1999, there was no Shared Registration System (SRS). Network Solutions (NSI) operated the .com, .net, and .org registries, and was the de jure registrar and domain name registry operator. However, several companies had set up as de facto registrars, including NetNames, who invented the idea of a commercial standalone domain name registration service in 1996. Registrars formed another link in the food chain, introducing the concept of domain name sales, effectively introducing the wholesale model into the industry. NSI followed suit, forcing the issue of separation of Registry and Registrar.
In October 1998, following pressure from the growing domain name registration business and other interested parties, NSI's agreement with the US Department of Commerce was amended, requiring the creation of an SRS that supported multiple registrars. The SRS officially opened on November 30, 1999 under the supervision of ICANN, though there had been several testbed registrars using the system since March 11, 1999. Since then, over 500 registrars have entered the market for domain name registration services.
Designated registrar
An end-user cannot directly register and manage their domain name information with ICANN. A designated registrar must be chosen to have one's domain names registered and managed with the appropriate registry (NIC) on their behalf. Prior to 1999, the only .com registrar was NSI, but the approval of the SRS opened up the opportunity for other companies to be designated as registrars.
Each ICANN-accredited registrar must pay a fixed fee of US$4,000 plus a per-registrar variable fee totaling US$3.8 million divided among all registrars.
Only one designated registrar may modify or delete information about a domain name. The competition that SRS created enables the end user to choose from many registrars offering different services at varying prices. It is not unusual for an end user to wish to switch registrars. Thus, there is the domain name transfer clause.
When a registrar registers a .com domain name for the end-user, it must pay a maximum annual fee of US$6.00 to VeriSign and a US$0.20 administration fee to ICANN. VeriSign is the registry manager for .com gTLD. Low cost bulk registrars like Go Daddy and Tucows must manage their margin after paying these fees and their equipment cost. Therefore, the barrier for entry into the bulk registrar industry is high for new companies without an existing customer base.
An end-user registers either directly with a registrar, or indirectly through one or more layers of resellers. The cost generally ranges from a low of about $10 per year to about $30 per year, as of 2008. The maximum period of registration is generally 10 years ahead. [1] [2] [3] Some registrars are offering longer periods, up to one hundred years, but such services are implemented internally, by promising to renew annually, not in the official registration database. Some packages of services, such as web hosting, include the domain registration in the total package pricing.
Domain name transfers
Domain name transfers is the act of designating a new registrar with the authority to add, modify, and delete information about the domain name. The usual process of a domain name transfer is:
The end user verifies that the whois admin contact info is correct, particularly the email address; obtains the authentication code from the old registrar, and removes any lock that has been placed on the registration.
The end user contacts the new registrar with the wish to transfer the domain name to their service, and supplies the authentication code.
The new registrar will contact the old registrar with this information.
The old registrar will contact the end user to confirm the authenticity of this request. The end user may have to take further action with the old registrar, such as returning to the online management tools, to re-iterate their desire to proceed, in order to expedite the transfer.
The old registrar will release authority to the new registrar.
The new registrar will notify the end user of transfer completion. The new registrar may have automatically copied over the domain server information, and everything may automatically continue to work as before. Otherwise, the domain server information will need to be updated with the new registrar.
After this process, the new registrar becomes one's designated registrar and all correspondence shall be done with them. The process may take about five days. In some cases, the old registrar may intentionally delay the transfer as long as allowable. After transfer, the domain cannot be transferred again for 60 days, except back to the previous registrar.
It is unwise to attempt to transfer a domain immediately before it expires. Because a transfer can in some cases take up to 14 days, the transfer may not complete before the registration expires, resulting in loss of the domain name registration and failure of the transfer. To avoid this, either transfer well before the expiration date, or renew the registration before attempting the transfer.
Transfer scams
With the introduction of SRS, many smaller registrars had to compete with the de facto standard, NSI. Some companies offered value added services or used viral marketing. Some companies decided to trick customers to switch from NSI.
Many of these transfer scams involve a notice sent in the mail, fax, or e-mail. Some scammers may even call by phone (as the contact information is available through WHOIS) to harvest more information. These notices would include information publicly available from the WHOIS database to add to the look of authenticity. The text would include legalese to confuse the end user into thinking that it is an official binding document.
Scam registrars go after domain names that are expiring soon or have recently expired. Expired domain names do not have to go through the authentication process to be transferred, as the previous registrar would have relinquished management rights of the domain name. Domain name expiry dates are readily available via WHOIS.
The Internet Corporation for Assigned Names and Numbers, or ICANN, has authority over generic top-level domains, or gTLDs. Examples of gTLDs include .com, .net, .org and .mobi. ICANN does not have authority over ccTLDs, or Country Code Top-Level Domains, though it is quite common for domain name registrars to offer ccTLD registration services as well. Most registrars provide DNS hosting service, but this is not required, and is often considered a separate service.
Until 1999, there was no Shared Registration System (SRS). Network Solutions (NSI) operated the .com, .net, and .org registries, and was the de jure registrar and domain name registry operator. However, several companies had set up as de facto registrars, including NetNames, who invented the idea of a commercial standalone domain name registration service in 1996. Registrars formed another link in the food chain, introducing the concept of domain name sales, effectively introducing the wholesale model into the industry. NSI followed suit, forcing the issue of separation of Registry and Registrar.
In October 1998, following pressure from the growing domain name registration business and other interested parties, NSI's agreement with the US Department of Commerce was amended, requiring the creation of an SRS that supported multiple registrars. The SRS officially opened on November 30, 1999 under the supervision of ICANN, though there had been several testbed registrars using the system since March 11, 1999. Since then, over 500 registrars have entered the market for domain name registration services.
Designated registrar
An end-user cannot directly register and manage their domain name information with ICANN. A designated registrar must be chosen to have one's domain names registered and managed with the appropriate registry (NIC) on their behalf. Prior to 1999, the only .com registrar was NSI, but the approval of the SRS opened up the opportunity for other companies to be designated as registrars.
Each ICANN-accredited registrar must pay a fixed fee of US$4,000 plus a per-registrar variable fee totaling US$3.8 million divided among all registrars.
Only one designated registrar may modify or delete information about a domain name. The competition that SRS created enables the end user to choose from many registrars offering different services at varying prices. It is not unusual for an end user to wish to switch registrars. Thus, there is the domain name transfer clause.
When a registrar registers a .com domain name for the end-user, it must pay a maximum annual fee of US$6.00 to VeriSign and a US$0.20 administration fee to ICANN. VeriSign is the registry manager for .com gTLD. Low cost bulk registrars like Go Daddy and Tucows must manage their margin after paying these fees and their equipment cost. Therefore, the barrier for entry into the bulk registrar industry is high for new companies without an existing customer base.
An end-user registers either directly with a registrar, or indirectly through one or more layers of resellers. The cost generally ranges from a low of about $10 per year to about $30 per year, as of 2008. The maximum period of registration is generally 10 years ahead. [1] [2] [3] Some registrars are offering longer periods, up to one hundred years, but such services are implemented internally, by promising to renew annually, not in the official registration database. Some packages of services, such as web hosting, include the domain registration in the total package pricing.
Domain name transfers
Domain name transfers is the act of designating a new registrar with the authority to add, modify, and delete information about the domain name. The usual process of a domain name transfer is:
The end user verifies that the whois admin contact info is correct, particularly the email address; obtains the authentication code from the old registrar, and removes any lock that has been placed on the registration.
The end user contacts the new registrar with the wish to transfer the domain name to their service, and supplies the authentication code.
The new registrar will contact the old registrar with this information.
The old registrar will contact the end user to confirm the authenticity of this request. The end user may have to take further action with the old registrar, such as returning to the online management tools, to re-iterate their desire to proceed, in order to expedite the transfer.
The old registrar will release authority to the new registrar.
The new registrar will notify the end user of transfer completion. The new registrar may have automatically copied over the domain server information, and everything may automatically continue to work as before. Otherwise, the domain server information will need to be updated with the new registrar.
After this process, the new registrar becomes one's designated registrar and all correspondence shall be done with them. The process may take about five days. In some cases, the old registrar may intentionally delay the transfer as long as allowable. After transfer, the domain cannot be transferred again for 60 days, except back to the previous registrar.
It is unwise to attempt to transfer a domain immediately before it expires. Because a transfer can in some cases take up to 14 days, the transfer may not complete before the registration expires, resulting in loss of the domain name registration and failure of the transfer. To avoid this, either transfer well before the expiration date, or renew the registration before attempting the transfer.
Transfer scams
With the introduction of SRS, many smaller registrars had to compete with the de facto standard, NSI. Some companies offered value added services or used viral marketing. Some companies decided to trick customers to switch from NSI.
Many of these transfer scams involve a notice sent in the mail, fax, or e-mail. Some scammers may even call by phone (as the contact information is available through WHOIS) to harvest more information. These notices would include information publicly available from the WHOIS database to add to the look of authenticity. The text would include legalese to confuse the end user into thinking that it is an official binding document.
Scam registrars go after domain names that are expiring soon or have recently expired. Expired domain names do not have to go through the authentication process to be transferred, as the previous registrar would have relinquished management rights of the domain name. Domain name expiry dates are readily available via WHOIS.
Hostname
A hostname (occasionally also, a sitename) is the unique name by which a network-attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc.) is known on a network. The hostname is used to identify a particular host in various forms of electronic communication such as the World Wide Web, e-mail or Usenet.
On the Internet, the terms "hostname" and "domain name" are often used interchangeably, but there are subtle technical differences between them.
Hostnames are used by various naming systems, NIS, DNS, SMB, etc., and so the meaning of the word hostname will vary according to the naming system in question, which in turn varies by type of network. A hostname meaningful to a Microsoft NetBIOS workgroup may be an invalid Internet hostname. When presented with a hostname and no context, it is usually safe to assume that the network is the Internet and DNS is the hostname's naming system.
Host names are typically used in an administrative capacity and may appear in computer browser lists, active directory lists, IP address to hostname resolutions, email headers, etc. They are human-readable nicknames, which ultimately correspond to unique network hardware MAC addresses. In some cases the host name may contain embedded domain names and/or locations, non-dotted IP addresses, etc.
On a simple local area network, a hostname is usually a single word: for instance, an organization's CVS server might be named "cvs" or "server-1".
Internet hostnames
On the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, "en.wikipedia.org" consists of a local hostname ("en") and the domain name "wikipedia.org". This kind of hostname is translated into an IP address via the local hosts file, or the Domain Name System (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.
Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. So, for example, both "en.wikimedia.org" and "wikimedia.org" are hostnames because they both have IP addresses assigned to them. The domain name "pmtpa.wikimedia.org" is not a hostname since it does not have an IP address, but "rr.pmtpa.wikimedia.org" is a hostname. All hostnames are domain names, but not all domain names are hostnames.
Restrictions on valid host names
Hostnames, like all domain names[1], are made up of a series of labels, with each label being separated by a dot. Each label must be between 1 and 63 characters long, and there is a maximum of 255 characters when all labels are combined.
Unlike domain names, hostname labels can only be made up of the ASCII letters 'a' through 'z' (case-insensitive), the digits '0' through '9', and the hyphen. Labels cannot start nor end with a hyphen. Special characters other than the hyphen (and the dot between labels) are not allowed, although they are sometimes used anyway. Underscore characters are commonly used by Windows systems but according to RFC 952 they are not allowed and several systems, such as DomainKeys and the SRV record deliberately use the underscore to make sure their special domain names are not confused with a hostname. Since some systems will check to make sure that hostnames contain only valid characters and others do not, the use of the invalid characters such as the underscore has caused many subtle problems in systems that connect to the wider world.
So, the hostname "en.wikipedia.org" is made up of the DNS labels "en", "wikipedia" and "org". Labels such as "2600" and "3com" can be used in hostnames, but "-hi-" and "*hi*" are invalid.
A hostname is considered to be a fully qualified domain name (FQDN) if all the labels up to and including the top-level domain name (TLD) are specified. Depending on the system, an unqualified hostname such as "compsci" or "wikipedia" may be combined with default domain names in order to determine the fully qualified domain name. So, a student at Harvard may be able to send mail to "joe@compsci" and have it sent to compsci.harvard.edu.
Choosing host names
General guidelines on choosing good hostnames are outlined in RFC 1178. The folklore interest of hostnames stems from the creativity and humour they often display. Interpreting a sitename is not unlike interpreting a vanity licence plate; one has to mentally unpack it, allowing for mono-case and length restrictions and the lack of whitespace. Hacker tradition deprecates dull, institutional-sounding names in favour of punchy, humorous, and clever coinages (except that it is considered appropriate for the official public gateway machine of an organisation to bear the organisation's name or acronym). Mythological references, cartoon characters, animal names, and allusions to sci-fi or fantasy literature are probably the most popular sources for sitenames (in roughly descending order).
On the Internet, the terms "hostname" and "domain name" are often used interchangeably, but there are subtle technical differences between them.
Hostnames are used by various naming systems, NIS, DNS, SMB, etc., and so the meaning of the word hostname will vary according to the naming system in question, which in turn varies by type of network. A hostname meaningful to a Microsoft NetBIOS workgroup may be an invalid Internet hostname. When presented with a hostname and no context, it is usually safe to assume that the network is the Internet and DNS is the hostname's naming system.
Host names are typically used in an administrative capacity and may appear in computer browser lists, active directory lists, IP address to hostname resolutions, email headers, etc. They are human-readable nicknames, which ultimately correspond to unique network hardware MAC addresses. In some cases the host name may contain embedded domain names and/or locations, non-dotted IP addresses, etc.
On a simple local area network, a hostname is usually a single word: for instance, an organization's CVS server might be named "cvs" or "server-1".
Internet hostnames
On the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, "en.wikipedia.org" consists of a local hostname ("en") and the domain name "wikipedia.org". This kind of hostname is translated into an IP address via the local hosts file, or the Domain Name System (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.
Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. So, for example, both "en.wikimedia.org" and "wikimedia.org" are hostnames because they both have IP addresses assigned to them. The domain name "pmtpa.wikimedia.org" is not a hostname since it does not have an IP address, but "rr.pmtpa.wikimedia.org" is a hostname. All hostnames are domain names, but not all domain names are hostnames.
Restrictions on valid host names
Hostnames, like all domain names[1], are made up of a series of labels, with each label being separated by a dot. Each label must be between 1 and 63 characters long, and there is a maximum of 255 characters when all labels are combined.
Unlike domain names, hostname labels can only be made up of the ASCII letters 'a' through 'z' (case-insensitive), the digits '0' through '9', and the hyphen. Labels cannot start nor end with a hyphen. Special characters other than the hyphen (and the dot between labels) are not allowed, although they are sometimes used anyway. Underscore characters are commonly used by Windows systems but according to RFC 952 they are not allowed and several systems, such as DomainKeys and the SRV record deliberately use the underscore to make sure their special domain names are not confused with a hostname. Since some systems will check to make sure that hostnames contain only valid characters and others do not, the use of the invalid characters such as the underscore has caused many subtle problems in systems that connect to the wider world.
So, the hostname "en.wikipedia.org" is made up of the DNS labels "en", "wikipedia" and "org". Labels such as "2600" and "3com" can be used in hostnames, but "-hi-" and "*hi*" are invalid.
A hostname is considered to be a fully qualified domain name (FQDN) if all the labels up to and including the top-level domain name (TLD) are specified. Depending on the system, an unqualified hostname such as "compsci" or "wikipedia" may be combined with default domain names in order to determine the fully qualified domain name. So, a student at Harvard may be able to send mail to "joe@compsci" and have it sent to compsci.harvard.edu.
Choosing host names
General guidelines on choosing good hostnames are outlined in RFC 1178. The folklore interest of hostnames stems from the creativity and humour they often display. Interpreting a sitename is not unlike interpreting a vanity licence plate; one has to mentally unpack it, allowing for mono-case and length restrictions and the lack of whitespace. Hacker tradition deprecates dull, institutional-sounding names in favour of punchy, humorous, and clever coinages (except that it is considered appropriate for the official public gateway machine of an organisation to bear the organisation's name or acronym). Mythological references, cartoon characters, animal names, and allusions to sci-fi or fantasy literature are probably the most popular sources for sitenames (in roughly descending order).
Website
A website (alternatively, web site or Web site) is a collection of Web pages, images, videos or other digital assets that is hosted on one or more web servers, usually accessible via the Internet.
A Web page is a document, typically written in HTML, that is almost always accessible via HTTP, a protocol that transfers information from the Web server to display in the user's Web browser.
All publicly accessible websites are seen collectively as constituting the "World Wide Web".
The pages of websites can usually be accessed from a common root URL called the homepage, and usually reside on the same physical server. The URLs of the pages organize them into a hierarchy, although the hyperlinks between them control how the reader perceives the overall structure and how the traffic flows between the different parts of the sites.
Some websites require a subscription to access some or all of their content. Examples of subscription sites include many business sites, parts of many news sites, academic journal sites, gaming sites, message boards, Web-based e-mail, services, social networking websites, and sites providing real-time stock market data.
Organized by function a website may be
a personal website
a commercial website
a government website
a non-profit organization website
It could be the work of an individual, a business or other organization and is typically dedicated to some particular topic or purpose. Any website can contain a hyperlink to any other website, so the distinction between individual sites, as perceived by the user, may sometimes be blurred.
Websites are written in, or dynamically converted to, HTML (Hyper Text Markup Language) and are accessed using a software interface classified as an user agent. Web pages can be viewed or otherwise accessed from a range of computer-based and Internet-enabled devices of various sizes, including desktop computers, laptop computers, PDAs and cell phones.
A website is hosted on a computer system known as a web server, also called an HTTP server, and these terms can also refer to the software that runs on these system and that retrieves and delivers the Web pages in response to requests from the website users. Apache is the most commonly used Web server software (according to Netcraft statistics) and Microsoft's Internet Information Server (IIS) is also commonly used.
[edit] Website styles
A static website is one that has web pages stored on the server in the same form as the user will view them. They are edited using three broad categories of software:
Text editors. such as Notepad or TextEdit, where the HTML is manipulated directly within the editor program
WYSIWYG editors. such as Microsoft FrontPage and Adobe Dreamweaver (previously Macromedia Dreamweaver), where the site is edited using a GUI interface and the underlying HTML is generated automatically by the editor software
Template-based editors, such as Rapidweaver and iWeb, which allow users to quickly create and upload websites to a web server without having to know anything about HTML, as they just pick a suitable template from a palette and add pictures and text to it in a DTP-like fashion without ever having to see any HTML code.
A dynamic website is one that has frequently changing information or collates information on the hop each time a page is requested. For example, it would call various bits of information from a database and put them together in a pre-defined format to present the reader with a coherent page. It interacts with users in a variety of ways including by reading cookies recognizing users' previous history, session variables, server side variables etc., or by using direct interaction (form elements, mouseovers, etc.). A site can display the current state of a dialogue between users, monitor a changing situation, or provide information in some way personalized to the requirements of the individual user.
Some countries, for example the U.K. have introduced legislation regarding web accessibility [1].
[edit] Software systems
There is a wide range of software systems, such as Java Server Pages (JSP), the PHP and Perl programming languages, Active Server Pages (ASP) and ColdFusion (CFM) that are available to generate dynamic Web systems and dynamic sites. Sites may also include content that is retrieved from one or more databases or by using XML-based technologies such as RSS.
Static content may also be dynamically generated either periodically, or if certain conditions for regeneration occur (cached) in order to avoid the performance loss of initiating the dynamic engine on a per-user or per-connection basis.
Plugins are available to expand the features and abilities of Web browsers, which use them to show active content, such as Flash, Shockwave or applets written in Java. Dynamic HTML also provides for user interactivity and realtime element updating within Web pages (i.e., pages don't have to be loaded or reloaded to effect any changes), mainly using the DOM and JavaScript, support which is built-in to most modern Web browsers.
website
Turning a website into an income source is a common practice for web-developers and website owners. There are several methods for creating a website business which fall into two broad categories, as defined below.
1. Content based sites
Some websites derive revenue by selling advertising space on the site (see contextual ads).
2. Product or service based sites
Some websites derive revenue by offering products or services. In the case of e-commerce websites, the products or services may be purchased at the website itself, by entering credit card or other payment information into a payment form on the site. While most business websites serve as a shop window for existing brick and mortar businesses, it is increasingly the case that some websites are businesses in their own right; that is, the products they offer are only available for purchase on the web.
Guides have been published which explain how to create a variety of types of websites including those in both the above categories. See the links at the bottom of this page.
Websites occasionally derive income from a combination of these two practices. For example, a website such as an online auctions website may charge the users of its auction service to list an auction, but also display third-party advertisements on the site, from which it derives further income.
A Web page is a document, typically written in HTML, that is almost always accessible via HTTP, a protocol that transfers information from the Web server to display in the user's Web browser.
All publicly accessible websites are seen collectively as constituting the "World Wide Web".
The pages of websites can usually be accessed from a common root URL called the homepage, and usually reside on the same physical server. The URLs of the pages organize them into a hierarchy, although the hyperlinks between them control how the reader perceives the overall structure and how the traffic flows between the different parts of the sites.
Some websites require a subscription to access some or all of their content. Examples of subscription sites include many business sites, parts of many news sites, academic journal sites, gaming sites, message boards, Web-based e-mail, services, social networking websites, and sites providing real-time stock market data.
Organized by function a website may be
a personal website
a commercial website
a government website
a non-profit organization website
It could be the work of an individual, a business or other organization and is typically dedicated to some particular topic or purpose. Any website can contain a hyperlink to any other website, so the distinction between individual sites, as perceived by the user, may sometimes be blurred.
Websites are written in, or dynamically converted to, HTML (Hyper Text Markup Language) and are accessed using a software interface classified as an user agent. Web pages can be viewed or otherwise accessed from a range of computer-based and Internet-enabled devices of various sizes, including desktop computers, laptop computers, PDAs and cell phones.
A website is hosted on a computer system known as a web server, also called an HTTP server, and these terms can also refer to the software that runs on these system and that retrieves and delivers the Web pages in response to requests from the website users. Apache is the most commonly used Web server software (according to Netcraft statistics) and Microsoft's Internet Information Server (IIS) is also commonly used.
[edit] Website styles
A static website is one that has web pages stored on the server in the same form as the user will view them. They are edited using three broad categories of software:
Text editors. such as Notepad or TextEdit, where the HTML is manipulated directly within the editor program
WYSIWYG editors. such as Microsoft FrontPage and Adobe Dreamweaver (previously Macromedia Dreamweaver), where the site is edited using a GUI interface and the underlying HTML is generated automatically by the editor software
Template-based editors, such as Rapidweaver and iWeb, which allow users to quickly create and upload websites to a web server without having to know anything about HTML, as they just pick a suitable template from a palette and add pictures and text to it in a DTP-like fashion without ever having to see any HTML code.
A dynamic website is one that has frequently changing information or collates information on the hop each time a page is requested. For example, it would call various bits of information from a database and put them together in a pre-defined format to present the reader with a coherent page. It interacts with users in a variety of ways including by reading cookies recognizing users' previous history, session variables, server side variables etc., or by using direct interaction (form elements, mouseovers, etc.). A site can display the current state of a dialogue between users, monitor a changing situation, or provide information in some way personalized to the requirements of the individual user.
Some countries, for example the U.K. have introduced legislation regarding web accessibility [1].
[edit] Software systems
There is a wide range of software systems, such as Java Server Pages (JSP), the PHP and Perl programming languages, Active Server Pages (ASP) and ColdFusion (CFM) that are available to generate dynamic Web systems and dynamic sites. Sites may also include content that is retrieved from one or more databases or by using XML-based technologies such as RSS.
Static content may also be dynamically generated either periodically, or if certain conditions for regeneration occur (cached) in order to avoid the performance loss of initiating the dynamic engine on a per-user or per-connection basis.
Plugins are available to expand the features and abilities of Web browsers, which use them to show active content, such as Flash, Shockwave or applets written in Java. Dynamic HTML also provides for user interactivity and realtime element updating within Web pages (i.e., pages don't have to be loaded or reloaded to effect any changes), mainly using the DOM and JavaScript, support which is built-in to most modern Web browsers.
website
Turning a website into an income source is a common practice for web-developers and website owners. There are several methods for creating a website business which fall into two broad categories, as defined below.
1. Content based sites
Some websites derive revenue by selling advertising space on the site (see contextual ads).
2. Product or service based sites
Some websites derive revenue by offering products or services. In the case of e-commerce websites, the products or services may be purchased at the website itself, by entering credit card or other payment information into a payment form on the site. While most business websites serve as a shop window for existing brick and mortar businesses, it is increasingly the case that some websites are businesses in their own right; that is, the products they offer are only available for purchase on the web.
Guides have been published which explain how to create a variety of types of websites including those in both the above categories. See the links at the bottom of this page.
Websites occasionally derive income from a combination of these two practices. For example, a website such as an online auctions website may charge the users of its auction service to list an auction, but also display third-party advertisements on the site, from which it derives further income.
The term domain name has multiple related meanings:
The term domain name has multiple related meanings:
A name that identifies a computer or computers on the Internet. These names appear as a component of a Web site's URL, e.g. en.wikipedia.org. This type of domain name is also called a hostname.
The product that domain name registrars provide to their customers. These names are often called registered domain names.
Names used for other purposes in the Domain Name System (DNS), for example the special name which follows the @ sign in an email address, or the Top-level domain names like .com, or the names used by the Session Initiation Protocol (VoIP), or DomainKeys.
They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses".
This article will primarily discuss registered domain names. See the Domain Name System article for technical discussions about general domain names and the hostname article for further information about the most common type of domain name.
A name that identifies a computer or computers on the Internet. These names appear as a component of a Web site's URL, e.g. en.wikipedia.org. This type of domain name is also called a hostname.
The product that domain name registrars provide to their customers. These names are often called registered domain names.
Names used for other purposes in the Domain Name System (DNS), for example the special name which follows the @ sign in an email address, or the Top-level domain names like .com, or the names used by the Session Initiation Protocol (VoIP), or DomainKeys.
They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses".
This article will primarily discuss registered domain names. See the Domain Name System article for technical discussions about general domain names and the hostname article for further information about the most common type of domain name.
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