In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of a main domain.
Contents
Overview
The Domain Name System (DNS) has a tree structure or hierarchy, with each non-RR (resource record) node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain. For example, west.example.com
and east.example.com
are subdomains of the example.com
domain, which in turn is a subdomain of the com
top-level domain (TLD). A "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 most domain registries limit at 253 characters.
Subdomains in this context are defined by editing the DNS zone file pertaining to the parent domain. However, there is a lively debate over the use of the term “subdomain” when referring to names which map to the Address record (A; host) and various other types of zone records which may map to any public IP address destination and any type of server. Certain groups insist that it is inappropriate to use the term “subdomain” to refer to any mapping other than that provided by zone NS (name server) records and any server-destination other than that of a domain name server. Notwithstanding the terminology debate, many prominent public DNS providers use the term “subdomain” to refer to names which map to A (host) records which may map to any type of host or destination-server.
Uses
In the United Kingdom, the third-level domain names are standard and branch off from the second-level domains. For example:
Subdomains are also 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
.
Vanity domain
A vanity domain is a subdomain of an ISP's domain that is aliased to an individual user account, or a subdomain that expresses the individuality of the person on whose behalf it is registered.
Server cluster
Depending on application, a record inside a domain, or subdomain might refer to a hostname, or a service provided by a number of machines in a cluster. Some websites use 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..
Some domains host their nameservers as ns1.example.com, ns2.example.com, etc., and these usually never show up in search engine results.
Subdomains versus directories
Subdomains are different from directories. For example, example.com/yn points to a directory within the example.com domain, not to a subdomain of example.com.