nxx

NXX is a term used in the North American Numbering Plan (NANP) to designate the three-digit central office code that identifies a specific local telephone exchange within a given area code. The NXX forms the middle three digits of a standard ten-digit NANP number, with the area code determining the geographic region and the final four digits identifying the subscriber line.

The format NXX uses digits where N is any digit from 2 through 9 and X is any digit from 0 through 9. This means central office codes cannot begin with 0 or 1. The central office code is assigned to a telephone switch or exchange by the North American Numbering Plan Administrator (NANPA) and is used by the network to route calls to the correct local carrier and switch.

In practice, NXX values, together with the area code, create a six-digit local routing prefix. The remaining four digits route to a specific subscriber line. Over time, central office code exhaustion has led to area-code overlays and split plans, as demand for numbers rises. Carriers may port NXX blocks between service providers as part of number portability and competitive provisioning.

Some central office codes are reserved or have special uses; for example, the 555 central office code is widely restricted in real-world numbers and is often used for fictional or illustrative purposes in media. See also North American Numbering Plan, NANPA, and E.164 for related numbering concepts.

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