The DNS specifications attempt to be as general as possible in the rules for constructing domain names. The idea is that the name of any existing object can be expressed as a domain name with minimal changes. However, when assigning a domain name for an object, the prudent user will select a name which satisfies both the rules of the domain system and any existing rules for the object, whether these rules are published or implied by existing programs.
For example, when naming a mail domain, the user should satisfy both the rules of this memo and those in RFC-822. When creating a new host name, the old rules for HOSTS.TXT should be followed. This avoids problems when old software is converted to use domain names.
The following syntax will result in fewer problems with many applications that use domain names (e.g., mail, TELNET).
|<domain>||::=||<subdomain> | " "|
|<subdomain>||::=||<label> | <subdomain> "." <label>|
|<label>||::=||<letter> [ [ <ldh-str> ] <let-dig> ]|
|<ldh-str>||::=||<let-dig-hyp> | <let-dig-hyp> <ldh-str>|
|<let-dig-hyp>||::=||<let-dig> | "-"|
|<let-dig>||::=||<letter> | <digit>|
|<letter>||::=||any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case|
|<digit>||::=||any one of the ten digits 0 through 9|
Note that while upper and lower case letters are allowed in domain names, no significance is attached to the case. That is, two names with the same spelling but different case are to be treated as if identical.
The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphen. There are also some restrictions on the length. Labels must be 63 characters or less.
For example, the following strings identify hosts in the Internet: