There are three ccTLDs that have been deleted after the corresponding 2-letter code was withdrawn from ISO 3166-1: cs (for Czechoslovakia), zr (for Zaire) and tp (for East Timor). There may be a significant delay between withdrawal from ISO 3166-1 and deletion from the DNS; for example, ZR ceased to be an ISO 3166-1 code in 1997, but the zr ccTLD was not deleted until 2001. Other ccTLDs corresponding to obsolete ISO 3166-1 codes have not yet been deleted. In some cases they may never be deleted due to the amount of disruption this would cause for a heavily used ccTLD. In particular, the Soviet Union’s ccTLD su remains in use more than twenty years after SU was removed from ISO 3166-1.

The historical country codes dd for the German Democratic Republic and yd for South Yemen were eligible for a ccTLD, but not allocated;

The temporary reassignment of country code cs (Serbia and Montenegro) until its split into rs and me (Serbia and Montenegro, respectively) led to some controversies about the stability of ISO 3166-1 country codes, resulting in a second edition of ISO 3166-1 in 2007 with a guarantee that retired codes will not be reassigned for at least 50 years, and the replacement of RFC 3066 by RFC 4646 for country codes used in language tags in 2006.

The previous ISO 3166-1 code for Yugoslavia, YU, was removed by ISO on 2003-07-23, but the yu ccTLD remained in operation. Finally, after a two-year transition to Serbian rs and Montenegrin me, the .yu domain was phased out in March 2010.

Australia was originally assigned the oz country code, which was later changed to au with the .oz domains moved to .oz.au.

Internationalized ccTLDs

An internationalized country code top-level domain (IDN ccTLD) is a top-level domain with a specially encoded domain name that is displayed in an end user application, such as a web browser, in its language-native script or alphabet, such as the Arabic alphabet, or a non-alphabetic writing system, such as Chinese characters. IDN ccTLDs are an application of the internationalized domain name (IDN) system to top-level Internet domains assigned to countries, or independent geographic regions.

ICANN started to accept applications for IDN ccTLDs in November 2009, and installed the first set into the Domain Names System in May 2010. The first set was a group of Arabic names for the countries of Egypt, Saudi Arabia, and the United Arab Emirates. By May 2010, 21 countries had submitted applications to ICANN, representing 11 languages.

ICANN requires all potential international TLDs to use at least one letter that does not resemble a Latin letter, or have at least three letters, in an effort to avoid IDN homograph attacks. Nor shall the international domain name look like another domain name, even if they have different alphabets. Between Cyrillic and Greek alphabets, for example, this could happen.