[BCP42]    Best Current Practice 42,
              <https://www.rfc-editor.org/info/bcp42>.
              At the time of writing, this BCP comprises the following:

              Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.