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'.
- See also: RFC1034
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.
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'.
In this example:
- the top level domain is 'com'
- the second level domain is 'dreamhosters', and
- the subdomain is 'yoursite'.
Adding A Subdomain To Your DreamHost Account
- Navigate to the (Panel > 'Domains' > 'Manage Domains') page.
- Click the Add Hosting to a Domain / Sub-Domain button.
- On the next page, choose if you want this subdomain to be Fully Hosted, Redirected, Mirrored, Parked, or Cloaked.
Note: You cannot enter a subdomain in the “DNS Only” section.
- Enter the name of the subdomain you wish to add in the corresponding section.
- Click the blue button within each section to save the settings for the subdomain.