Valid Characters in Domain Names

The only characters that are valid for use in domain names are:

capital letters	A-Z
lowercase letters	a-z
numerals	0-9
hyphen	-
dot (period)	.

Note: In particular, the underscore character (_) is not legal!

You can require that hosts running both TCP/IP and DECnet Phase IV have identical hostname portions. Such hosts must conform to the more restrictive DECnet requirements for names:

six-character maximum

letters and digits only

must begin with a letter