NANPA — The North American Numbering Plan

01 What is NANPA?

NANPA stands for North American Numbering Plan Administration. It is the organization responsible for administering the North American Numbering Plan (NANP) — the telephone numbering scheme used across the United States, Canada, and numerous Caribbean nations.

The NANP itself has existed since the 1940s, when AT&T and the Bell System created a unified 10-digit numbering scheme to enable direct distance dialing without operator assistance. The formal administration role was separated from AT&T following the 1984 Bell System divestiture, and NANPA was established in 1984 as a neutral third-party administrator, currently operated under contract with the Federal Communications Commission (FCC) and the Canadian Radio-television and Telecommunications Commission (CRTC).

NANPA administers:

• NPA codes — Numbering Plan Areas, commonly called area codes
• NXX codes — Central Office Codes (exchanges) assigned to telephone carriers
• Thousands-blocks — 1000-number sub-divisions of NXX codes
• Special-purpose codes — N11 codes, toll-free numbers, premium-rate, and other reserved ranges
• CO Code Assignment data — publicly available records of which carrier holds each NXX in each NPA

02 The NPA-NXX-XXXX Structure

Every telephone number in the NANP is exactly 10 digits, formatted as NPA-NXX-XXXX. When dialed internationally, the country code +1 precedes these 10 digits.

NPA — Numbering Plan Area

The first three digits identify the Numbering Plan Area, commonly called the area code. The first digit must be 2 through 9 (0 and 1 are reserved for special signaling purposes). The second and third digits can be 0–9, giving a theoretical maximum of 800 area codes — though in practice many are reserved or unavailable.

Originally, area codes with a 0 or 1 in the middle digit indicated a state/province with a single area code, while a digit 2–9 in the middle indicated a region with multiple codes. This distinction became obsolete as numbers were exhausted and overlays were introduced.

NXX — Central Office Code (Exchange)

The middle three digits are the Central Office Code, also called the NXX or exchange. Like the NPA, the first digit must be 2 through 9. Each NXX is assigned to a specific telephone carrier (identified by their OCN — Operating Company Number) within a specific NPA. A single NPA can contain hundreds of NXX codes assigned to dozens of different carriers.

The NXX was historically tied to a physical central office switch, but in the modern era of number portability, an NXX's geographic association is an originating indicator only — the subscriber may be anywhere in the rate center.

XXXX — Subscriber Number

The final four digits uniquely identify the subscriber line within a given NXX. With 10,000 possible values (0000–9999), a single NXX code can theoretically serve up to 10,000 individual subscribers — though practical fill rates are typically far lower.

Total Capacity

Combining the constraints above, the NANP provides approximately 6.4 billion possible numbers (800 possible NPAs × 800 possible NXX codes per NPA × 10,000 subscriber numbers = 6,400,000,000). Not all are usable — reserved codes, special services, and inefficient assignment reduce the effective pool significantly, which is why number conservation measures like 1000-block pooling exist.

03 NANP Territory

The North American Numbering Plan is unique in that many independent nations share a single country code — +1. This is a legacy of the original Bell System network, which served North America as a unified telephone territory before national borders were a concern for numbering plans.

NANP territory includes the United States, Canada, and 17 Caribbean nations and territories:

From an international dialing perspective, all these territories look identical — you dial +1 followed by a 10-digit number. This can cause confusion: calling a Caribbean number may incur international rates even though the number format is identical to a US domestic number.

04 How Area Codes Are Assigned

When a geographic area exhausts its available NXX codes within an existing NPA, NANPA — working with the FCC and state regulators — must introduce new numbering capacity. There are two primary methods:

The Assignment Process

Carriers petition NANPA for new NXX codes in an NPA. NANPA tracks utilization rates — when an NPA reaches approximately 10% remaining capacity, the relief planning process begins. A Numbering Relief Planning Committee (NRPC) is convened with the relevant state PUC, carriers, and NANPA to evaluate options.

All new area code assignments are ultimately subject to FCC approval in the US, and CRTC approval in Canada.

05 NXX Exchange Assignment

Within each NPA, individual NXX codes (exchanges) are assigned to telephone carriers who request them. The assignment process is managed by NANPA and is based on documented need.

OCN — Operating Company Number

Every telephone carrier is identified by a 4-character Operating Company Number (OCN), sometimes called a SPID (Service Provider Identification). OCNs are assigned by NECA (National Exchange Carrier Association). When an NXX is assigned, it is tied to a specific OCN, which tells you which carrier "owns" that exchange.

The OCN type code indicates what kind of carrier it is:

Rate Centers

NXX codes are assigned within specific rate centers — geographic billing zones used for determining local vs. long-distance call charges. Each rate center has a defined geographic area and a rate center name (often matching a city or town). An NXX is tied to a rate center even though Local Number Portability (LNP) allows the subscriber behind it to be physically elsewhere.

Rate center data is published by NECA and is a key component of call routing tables used by every telephone switch in North America.

06 1000-Block Number Pooling

Historically, NANPA assigned NXX codes in full 10,000-number blocks. Even if a carrier only needed a few hundred numbers, they received all 10,000 in the NXX. This was enormously wasteful — studies found that many NXX codes were less than 2% utilized when they were assigned.

In 2000, the FCC mandated Thousands-Block Number Pooling for all Tier 1 markets (and later extended more broadly). Under this system:

• An NXX code is first assigned to a block administrator (typically the ILEC or a pooling administrator)
• The 10,000 numbers are divided into ten 1000-number blocks (block 0: XXXX 0000–0999, block 1: 1000–1999, etc.)
• Individual carriers request blocks from the pool as they need numbers
• Different blocks within the same NXX may be assigned to different carriers

This is why a lookup on a number in the 202-555-XXXX range might show a different carrier for 202-555-0xxx vs. 202-555-1xxx — they are different 1000-blocks assigned to different OCNs.

07 Special NPA & Service Codes

Not all NPAs correspond to geographic regions. NANP reserves several NPA ranges for special services, accessible from anywhere in the NANP territory:

Reserved and Unavailable NPAs

Several NPA ranges are permanently reserved and never assigned to geographic areas or services:

• X11 patterns (211, 311, etc.) — used as N11 service codes, not area codes
• X00 and X01 patterns — reserved to prevent confusion with country codes and special dialing sequences
• 700 — reserved for interexchange carrier identification services (largely unused)
• 500 — Personal Communications Services (follow-me numbers); limited deployment

08 NANPA Public Data

NANPA publishes several datasets that are freely available to the public and are the foundation for phone lookup tools, call routing systems, and telecommunications research:

The CO Code Assignment file is a cornerstone dataset. It is distributed as a TAB-delimited flat file with a combined NPA_NXX column (e.g. "617-555") and is updated regularly. Telecommunications carriers, phone lookup services, and routing systems use this data to map NPA-NXX combinations to carriers and rate centers.

The official source for all NANPA data: www.nanpa.com

09 How foneinfo.us Uses NANPA Data

foneinfo.us ingests the NANPA CO Code Assignment data and 1000-block pooling records to build the core of every phone lookup result. When you look up a number on foneinfo.us, the NPA-NXX is matched against the NANPA assignment database to determine:

• Carrier name — the OCN holder for that NPA-NXX (or 1000-block)
• OCN — the 4-character Operating Company Number
• Carrier type — ILEC, CLEC, Wireless, etc. (from OCN type codes)
• Rate center — the billing rate center for that exchange
• State / jurisdiction — derived from NPA and rate center records
• Line type — whether the number appears to be wireline, wireless, or VoIP based on carrier classification

This NANPA-derived data is combined with other sources (CNAM databases, LNP query results, CLLI data) to give you the most complete picture possible of any North American phone number.

Want to learn more about related topics? See our guides on Carrier Types (ILEC, CLEC, RBOC) and check the Learn Hub for the full reference library.