NPA - technical definition

The administration of the scheme of logical addresses, i.e., telephone numbers, used in the global switched telephone network (GSTN) and the national public switched telephone networks (PSTNs) that compose it.The ITU-T is responsible for international numbering plan administration, and individual nations or regions have similar responsibilities within their domains.The North American Numbering Plan (NANP), for example, defines the telephone numbering scheme in the area loosely described as North America.The ITU-T E.164 recommendation (The International Public Telecommunication Numbering Plan) specifies the current international NPA convention at a maximum of 15 digits, although the number of digits required for calling within a nation varies. In many cases, numbering schemes vary within the same country; for example, six- and seven-digit telephone numbers coexist in Namibia and many other countries. In the countries within the NANP, the dialing scheme is +CC.NPA.NXX.xxxx, with the fields defined as follows: