Subdomain

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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) and "www.example.com" is a subdomain of "example.com". In fact, the "com" TLD is a subdomain of the root domain, ".".

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 different www# to point to different server clusters. For example, www.domain.com points to Server Cluster 1 or Datacentre 1, and www2.domain.com points to Server Cluster 2 or Datacentre 2, etc.