The encoding of any identifier used as part of a Fully Qualifed Domain Name (FQDN) shall follow the Name Syntax
defined in RFC 2181 (Clarifications to the DNS Specification), RFC 1035 (Domain names - implementation and specification) and RFC 1123 (Requirements for Internet Hosts - Application and Support). 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 RFC 1035 the labels shall consist only of the alphabetic characters (A‑Z and a‑z), digits (0‑9) and
the hyphen (-).
Following RFC 1123, 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.