Q: How was the country code system developed?
A: In the early 1960s, a global numbering plan was devised so that the various national telephone systems can be linked; this used country codes of one to three digits in length, assigned according to geographic regions on the Earth. In fact, the system was developed from a numbering plan devised in Europe. International Telecommunications Union (ITU) documents from that time showed a numbering plan of two-digit country codes covering Europe and the Mediterranean Basin countries and even described at that time the overseas access codes to be used in various countries (France 19, UK 010 - most of these codes are still in use today). Many country codes from that original numbering plan were used in the worldwide plan such as France 33, UK 44 although many codes had to be renumbered for the new worldwide plan.
The world numbering zones (with initial country code digits) are:
There are a few anomalies to the zoning; St Pierre & Miquelon, a French territory near the Canadian province of Newfoundland, was issued a country code in zone 5 (country code 508), since North America already has the country code 1, and there were no codes available in zones 3 or 4 (at the time of original assignment). There was room in world zone 5 for the code. Similarly, Greenland (country code 299) could not be fit into the European zones, thus 299 was a code that was available from a nearby zone.
The TELECOM Digest Archives has country code listings, including a detailed set which indicates area/STD codes used within country codes as they would be dialed in international dialing (excluding domestic inter-regional prefix digits).
ITU-T Rec.X.121 is a specifiction of a numbering system for geographical areas which is used in the X.25 network specification. ftp://ftp.isi.edu/in-notes/rfc1236.txt details the mapping of IP address to X.25 addresses (X.121)
See also: