2.1 Extended Host Name
hname = name *("." name) name = 1*63let-dig-hyp-und let-dig-hyp-und = ALPHA / DIGIT / UTF8-2 / UTF8-3 / UTF8-4 / "-" / "_"
where UTF8-2, UTF8-3, and UTF8-4 are as specified in [RFC3629] section 4. In addition, the entire extended host name MUST be at most 255 bytes long.
An implementation MAY<1> disallow a string where a substring constructed from the "name" rule does not contain at least one non-DIGIT character.