An NPA is a Numbering Plan Area. The first digit of an NPA indicates the general geographic region of the number, with the second and third digits indicating more specific areas within that region. The fourth and fifth digits indicate the specific exchange within that area. NPAs are used to route calls to their proper destination and are often referred to as area codes.
NPAs were originally assigned by AT&T in 1947 when direct distance dialing was first introduced in the United States. At that time, there were 86 NPAs in use. Today, there are nearly 300 NPAs in use across the country. Each NPA is made up of a certain number of exchanges, which are groups of five-digit numbers prefixed by 1-5 or 0 (for example 555-1234).
The vast majority of telephone numbers in service today have 10 digits: a three-digit area code followed by a seven-digit local number (1+NPA+NXX). However, some parts of the country still use seven-digit dialing for local calls (1+NXX), while other areas have implemented 11-digit dialing for all calls (1+area code+7DLC).