Third level domain

In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of a main domain.

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, comprises a subdomain of the org domain, and comprises a subdomain of the domain . In theory this subdivision can go 127 levels deep (though that limit is not in any published RFC). Per RFC 1035, 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 an ongoing 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 well-known 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.

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