[BCP51]    Best Current Practice 51,
              <https://www.rfc-editor.org/info/bcp51>.
              At the time of writing, this BCP comprises the following:

              Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              DOI 10.17487/RFC5771, March 2010,
              <https://www.rfc-editor.org/info/rfc5771>.