North American Numbering Plan (NANP)

The North American Numbering Plan (NANP) is an integrated telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Numbering Zone 1 and has the country code 1. Some North American countries, most notably Mexico, do not participate in the NANP.

History

From the Bell System's beginnings in 1876 and throughout the first part of the 20th century, telephone networks grew from essentially local or regional telephone systems. These systems expanded by growing their subscriber bases, as well as enlarging their service areas by implementing additional local exchanges that were interconnected by trunks. It was the responsibility of each local administration to devise telephone numbering plans that accommodated the local requirements and growth. As a result, the North American telephone service industry developed into an unorganized set of many differing local numbering systems. The diversity impeded the efficient operation and interconnection of exchanges into a nationwide system for long-distance telephone communication. By the 1940s, the Bell System set out to unify the various existing numbering plans to provide a unified, systematic concept for routing telephone calls across the nation, as a precursor to the eventual system-wide introduction of Direct Distance Dialing.

In October 1947, AT&T published the first nationwide numbering plan in coordination with the independent telephone operators. The plan divided most of North America into eighty-six numbering plan areas (NPAs). Each NPA was assigned a unique three-digit code, typically termed an NPA code or simply an area code. These codes were first used in Operator Toll Dialing by long-distance operators in establishing calls via trunks between toll offices.

As long distance (toll) switching systems were installed, area codes were used by these systems to direct calls to their destinations. The first customer-dialed direct call using an area code was made on November 10, 1951, from Englewood, New Jersey, to Alameda, California. Direct distance dialing (DDD) was introduced subsequently across the country. By the early 1960s, DDD had become commonplace in cities and most towns in the United States and Canada. By 1967, the number of assigned area codes had grown to 129.

The status of the network of the 1960s was reflected by a new name used in technical documentation: North American Integrated Network. By 1975, the numbering plan was referred to as the North American Numbering Plan, resulting in the well-known initialism NANP, as other countries sought or considered joining the standardization.

Office Code

In NANP numbering practice, an office code is a 3-digit number that specifies a particular telephone exchange within a numbering plan area (NPA).