[BCP32]    Best Current Practice 32,
              <https://www.rfc-editor.org/info/bcp32>.
              At the time of writing, this BCP comprises the following:

              Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, DOI 10.17487/RFC2606, June 1999,
              <https://www.rfc-editor.org/info/rfc2606>.