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Concept
Subdomain
Summary
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of another (main) domain. For example, if a domain offered an online store as part of their website example.com, it might use the subdomain shop.example.com .
The Domain Name System (DNS) has a tree structure or hierarchy, which includes nodes on the tree being a domain name. A subdomain is a domain that is part of a larger domain. Each label may contain from 0 to 63 octets. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.
Subdomains 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. 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 (name server) records and any server-destination other than that.
According to RFC 1034, "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 (Start of Authority).
Most domain registries only allocate a two-level domain name. Hosting services typically provide DNS Servers to resolve subdomains within that master domain.
A fully qualified domain name consists of multiple parts. For example, take the English Wikipedia domain en.wikipedia.org.
The en is a subdomain of wikipedia.org.
Although wikipedia.org is usually considered to be the domain name, wikipedia is actually a sub-domain of the org TLD (top level domain). Any fully qualified domain name can be a host or a subdomain.
A domain name that does not include any subdomains is known as an apex domain, root domain, or bare domain. For example, wikipedia.
Official source
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