In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. A DNS hierarchy consists of the root-level domain at the top, underneath which are the top-level domains, followed by second-level domains and finally subdomains.
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; the Free On-line Dictionary of Computing defines it as "A domain you register for the sole purpose of having your own domain so you can have an easily remembered URL and e-mail address"while the Jargon File defines it as "An Internet domain, particularly in the .com or .org top-level domains, apparently created for no reason other than boosting the creator's ego."[http://catb.org/~esr/jargon/html/V/vanity-domain.html The Sun iPlanet Messaging Server uses msgVanityDomain to set up a domain name for e-mail use which is not fully hosted. These uses apply primarily to regular second-level domain registrations rather than subdomains (Although, technically speaking, a second-level domain is actually a subdomain of its top-level domain, itself.).
Valid subdomains as described by RFC RFC 1034 may contain any letter or number as well as the '-' (hyphen), but must begin with an alphabet character. While both upper and lower case characters are valid, subdomains are case insensitive so 'ABC.com' is identical to 'abc.COM'.
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