[BCP20]    Best Current Practice 20,
              <https://www.rfc-editor.org/info/bcp20>.
              At the time of writing, this BCP comprises the following:

              Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
              ADDR.ARPA delegation", BCP 20, RFC 2317,
              DOI 10.17487/RFC2317, March 1998,
              <https://www.rfc-editor.org/info/rfc2317>.