Genius Advice

In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. For example, "example.com" is a subdomain of the "com" top-level domain (TLD) while "www.example.com" is a service 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).

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 Host name to point to different servers in a 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.