[BCP49]    Best Current Practice 49,
              <https://www.rfc-editor.org/info/bcp49>.
              At the time of writing, this BCP comprises the following:

              Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
              DOI 10.17487/RFC3152, August 2001,
              <https://www.rfc-editor.org/info/rfc3152>.