Section: 19.4.2 Fully Qualified Domain Names (FQDNs) 19.4.2.1 General
Content: The encoding of any identifier used as part of a Fully Qualifed Domain Name (FQDN) shall follow the Name Syntax defined in IETF RFC 2181 [18], IETF RFC 1035 [19] and IETF RFC 1123 [20]. An FQDN consists of one or more labels. Each label is coded as a one octet length field followed by that number of octets coded as 8 bit ASCII characters. Following IETF RFC 1035 [19] the labels shall consist only of the alphabetic characters (A-Z and a-z), digits (0-9) and the hyphen (-). Following IETF RFC 1123 [20], the label shall begin and end with either an alphabetic character or a digit. The case of alphabetic characters is not significant. Identifiers are not terminated by a length byte of zero. NOTE: A length byte of zero is added by the querying entity at the end of the FQDN before interrogating a DNS server. For the purpose of presentation, identifiers are usually displayed as a string in which the labels are separated by dots (e.g. "Label1.Label2.Label3").