Central Office Code NXX

NANP · also called CO code, prefix, exchange, central office code

The three-digit prefix after the area code (the "NXX" in NPA-NXX-XXXX), historically identifying a telephone switch.

The central-office code — universally written NXX — is the second three-digit block of a NANP number. Historically each NXX mapped to a single telephone switch (central office) serving a rate center. An NXX takes the form [2-9][0-9][0-9].

Central-office codes are assigned to service providers by the numbering administrator. A single NPA has roughly 792 assignable NXX values. Assignment records name the initial assignee — not necessarily the carrier serving a given number today. Under number pooling, providers increasingly receive individual thousands-blocks rather than whole NXX codes.

A number is country code + area code (NPA) + prefix (NXX) + a four-digit line number. The first digit of the four-digit line number — the seventh digit of the 10-digit national number, not counting the country code +1 — marks which pooled thousands-block of 1,000 numbers it belongs to.

Common misconception.

An NXX no longer reliably identifies one physical switch or one carrier. Number pooling splits NXX codes into thousands-blocks held by different providers, and portability means the assignee of record may not serve the number today.

See it in live dataNumberResearch

Every area code page on NumberResearch shows this in practice — assigned prefixes, thousands-block breakdown and exhaust forecast. For example: area code 626 ↗.