Subdomain

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.

Valid subdomains as described by 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'.

Length
The length of a particular subdomain label is restricted to 63 characters, the number of subdomain labels in a domain name is limited to 127, and the sum of the lengths of all labels must not exceed 255 characters.

Structure
Subdomains are separated by dots "." and are read from left to right, in descending specificity. Each consecutive subdomain contains all previous subdomains, and is contained by any subsequent subdomains. All subdomains are contained by the second-level domain, which is in turn contained by its top level domain.

All the parts of a domain name, once joined together -- the top-level domain, the second-level domain and the subdomain -- are called a fully qualified domain name. For example, domain.com is a fully qualified domain name; so is www.domain.com, sub.domain.com, mail.domain.com and my.really.long.fully.qualified.domain.com

Example
subbywubby.dreamhosters.com In this example the top level domain is 'com', the second level domain is 'dreamhosters', and the subdomain is 'subbywubby'.

Adding A Subdomain To Your DreamHost Account
Note: If you get an error message "Can't add domain: you don't have permission to add to subdomain.example.com", it means that the second level domain is not yet in the system. Sub-domains can only be created if its second level domain is created first (example.com) - even if you don't plan on hosting the second level domain at DreamHost.
 * 1) Log into the Manage Domains section of the Control Panel
 * 2) Click on the Add New Domain / Sub-Domain
 * 3) Enter the name of the subdomain you wish to add in the following space (e.g., subdomain.example.com).
 * 4) Choose whether or not you want this subdomain to be fully hosted (meaning that content is hosted by DreamHost, with your domain pointing to it), redirected, mirrored, parked or cloaked.