NPA-NXX
The first six digits of a North American number: the area code plus the three-digit exchange prefix.
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The first six digits of a North American number: the area code plus the three-digit exchange prefix.
NPA-NXX describes the first six digits of a North American Numbering Plan phone number. NPA is the Numbering Plan Area, better known as the area code. NXX is the central office code, or exchange prefix, that follows it.
Numbers are allocated to carriers in NPA-NXX blocks, historically in units of 10,000 and now often in thousand-blocks. Public tables map each block to the carrier it was originally assigned to and to a rate centre, which is a geographic billing area.
The catch is that these tables describe the original allocation. Once number portability entered the picture, an individual number inside a block can be served by any carrier at all. NPA-NXX is a useful starting point and a poor final answer — it cannot tell you current carrier or line type.
Related terms
- Number portability — The ability to keep a phone number when switching from one carrier to another.
- Carrier lookup — Querying network data to find which carrier currently serves a phone number.
- E.164 — The international standard format for phone numbers: +, country code, then the national number.