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.uk is the Internet country code top-level domain (ccTLD) for the United Kingdom of Great Britain and Northern Ireland. As of March 2012, it is the fourth most popular top-level domain worldwide (after .com, .de and .net), with over 10 million registrations.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In October 1984, RFC 920<ref>{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref> set out the creation of ccTLDs generally using country codes derived from the corresponding two-letter code in the ISO 3166-1 list. GB is the UK's ISO 3166 country code. However, the .uk domain had been created separately a few months before the compilation of the ISO-derived list.<ref>{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref> Consequently, .gb was never widely used and it is no longer possible to register under that domain.

New registrations directly under .uk have been accepted by Nominet since 10 June 2014 08:00 BST, however there is a reservation period for existing customers who already have a,,,, or domain to claim the corresponding .uk domain, which runs until 07:59 BST on 10 June 2019.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

.uk has used OpenDNSSEC since March 2010.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> sections