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.