To register a domain name under ".com", ".net", ".org", or one of the other generic top level domain names, we recommend using one of the ICANN accredited registrars. See the list at:
Please note that not all registrars support IPv6 for DNS server registrations yet (June 2016). So if you want to run your DNS servers on IPv6 addresses, you need to make sure that the registrar supports this.
We recently moved our own domain names from NameBargain.com (a Register.com subsidiary) to Name.com for exactly this reason.
To register a country specific domain name (such as ".uk" or ".it") see the IANA CCTLD contact list at: http://www.iana.org/cctld/cctld.htm