[BCP4]     Best Current Practice 4,
              <https://www.rfc-editor.org/info/bcp4>.
              At the time of writing, this BCP comprises the following:

              Nesser II, P., "An Appeal to the Internet Community to
              Return Unused IP Networks (Prefixes) to the IANA", BCP 4,
              RFC 1917, DOI 10.17487/RFC1917, February 1996,
              <https://www.rfc-editor.org/info/rfc1917>.