The Domain Name System has a tree structure or hierarchy, with each non-RR 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 comtop-level domain. 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. Each label may contain from 1 to 63 octets. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a dot. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation. Thus, when using a single character per label, the limit is 127 levels: 127 characters plus 126 dots have a total length of 253. In practice, some domain registries may have shorter limits. Subdomains in this context are defined by editing the DNSzone file pertaining to the parent domain. However, there is an ongoing debate over the use of the term “subdomain” when referring to names which map to the Address record A and various other types of zone records which may map to any public IP address destination and any type of server. Network Operations teams insist that it is inappropriate to use the term “subdomain” to refer to any mapping other than that provided by zone NS records and any server-destination other than that. According to , "a domain is a subdomain of another domain if it is contained within that domain". Based on that definition, a host cannot be a subdomain, only a domain can be a subdomain. A subdomain will also have a separate zone file with a SOA record. Merely looking at a URL will not tell you if the left most node is a host or a subdomain.
Uses
In the United Kingdom, the second-level domain names are standard and branch off from the top-level domain. 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 do not typically 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.
