code: def is_valid(self):
          
          length = len(self.fqdn)
          if self.fqdn.endswith('.'):
              length -= 1
          if length > 253:
              return False
          return bool(self.FQDN_REGEX.match(self.fqdn))
docs: True for a validated fully-qualified domain nam (FQDN), in full
          compliance with RFC 1035, and the "preferred form" specified in RFC
          3686 s. 2, whether relative or absolute.
  
          https://tools.ietf.org/html/rfc3696#section-2
          https://tools.ietf.org/html/rfc1035
  
          If and only if the FQDN ends with a dot (in place of the RFC1035
          trailing null byte), it may have a total length of 254 bytes, still it
          must be less than 253 bytes.