2.1 Extended Host Name

The extended host name syntax is a UTF-8 [RFC3629] string specified by the following ABNF [RFC5234].

 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.