.gb
The normal rule in the Domain Name System is that a top-level domain for a country is derived from the corresponding two-letter code in the ISO 3166-1 list. According to that list the code for the United Kingdom of Great Britain and Northern Ireland is GB, and the .gb top-level domain was therefore delegated. However, GB is a common abbreviation for Great Britain, which does not include Northern Ireland. At the time only a very limited number of countries were on the Internet (less than 6), so the UK Name applied to Jon Postel of IANA for .uk. This was granted, and all UK domain names are now issued as .uk not .gb..gb was actually used for a number of years. The only major site to use it was DERA at their address dra.hmg.gb. (HMG standing for Her Majesty's Government.) It is now disused.