Numbering Plan Areas (NPA)
NPA (Number Planning Area/Number Plan Area/Numbering Plan Area), also known simply as an Area Code, is the three-digit number that identifies the telephone service region. For example, if you have the phone number 555-101-1234, the NPA would be 555.
While the Area Code identifies the telephone service region, it’s possible for a single NPA to be present in more than one rate center. For example, the 305 area code in Florida is present in the following rate centers: Keys, Miami, Homestead, North Dade, and Perrine. If you order a number in NPA 305, you can get a telephone number in any of those rate centers.
Number Plan Areas are based on national, regional or global telephone numbering plans. They serve as official guides that prescribe how phone numbers should be formatted. These numbering plans are used for public as well as some private phone networks. For public phone networks, telephone numbering plans often assign numbers by geographical location – thus the ‘area’ in ‘number plan area.’
Number Plan Area (NPA)
A Number Plan Area (NPAs) is a telephone service territory established in conformity with guidelines laid out in the relevant telephone numbering plan. NPAs are a common feature of telephone numbering plans that delineate service territories by geographical location. Each NPA is assigned a code, known as a Number Plan Area code, whose length is determined by the telephone numbering plan it references. NPA codes are also known as National Destination Codes (NDCs) or, more commonly, area codes, which is simply the short form of the longer-winded ‘number plan area code.’
The NPA concept was devised by Bell System engineers in the early 1940s as part of a plan to replace operator toll dialing with direct subscriber-to-subscriber calling. It was eventually incorporated into the North American Numbering Plan (NANP) of 1947, the telephone numbering plan used in the United States, Canada and about 19 other countries.
The NANP divides service territories into Number Plan Areas, each of which it assigns a unique three-digit code. This code is prefixed to every phone number issued in the service territory covered by the NPA in question.
History of the Number Plan Area
The Number Plan Area concept owes its existence to engineers at Bell System. Ma Bell, as it was affectionately known, developed the concept as part of a broad plan to replace the dialing technology that existed at the time, which relied on human operators to route calls. This reliance on human operators seriously constrained the system’s ability to grow. To solve the problem, Bell System engineers decided to take human operators out of the equation by automating the system to make direct subscriber-to-subscriber calls possible.
Part of the solution involved developing a uniform way of assigning phone numbers to subscribers to make dialing and call routing easier – a telephone numbering plan, in other words. The first such plan, dubbed the Nationwide Numbering Plan, was unveiled in 1947. It divided North America into 86 distinct Numbering Plan Areas and assigned a three-digit code to each. States with more than one area code, such as New York, typically got NPA codes with a one as the second digit; it tended to be a zero for those with a single area code. Leaving out numbers higher than one in the second digit of these very first area codes meant there was room for growth in the decades to come. The first digit could be any number between two and nine; the last digit could be any number between zero and nine. In 1995, the rules were modified to allow the center digit to be any number – except nine. Why? Nine was reserved in case the three-digit code pool got exhausted and the code length needed to be increased to four digits.
Other factors that may have determined which area got what codes were population and, interestingly, how easy a code was to dial on a rotary phone. Larger population centers with high call volumes – cities such as New York and Los Angeles, for example – got area codes that required fewer clicks on these phones.
In 1975, the Nationwide Numbering Plan got a new name: the North American Numbering Plan. The reason for the name change was because plans were underway to expand the numbering plan beyond the United States and Canada. Currently, the plan is used by Anguilla, Antigua and Barbuda, the Bahamas, Barbados and the British Virgin Islands. Other countries that use the plan are the Cayman Islands, Dominica, the Dominican Republic, Grenada and Jamaica. Montserrat, Sint Maarten, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Trinidad and Tobago, and Turks and Caicos are also part of the NANP.
Initially, AT&T administered the NANP. When the Bell System broke up in 1982, responsibility for managing the plan shifted to the North American Numbering Plan Administrator (NANPA). The organization does not make policy; its role is limited to following policy directives and industry standards. Individual countries in the plan are responsible for managing numbering resources within their jurisdictions.
Mechanics of the Number Plan Area
The NANP, as we saw earlier, divides telephone service territories into numbering plan areas, or NPAs, and assigns to each a three-digit area code. This code forms the first part of a telephone number. The rest consists of a three-digit central office (CO) code, otherwise known as a prefix, which indicates a specific exchange or rate center, and a four-digit station number. Combined, the area code, office code and station number serve as the destination routing address in the public switched telephone network (PSTN). This format is often represented as NPA-NXX-XXXX where NPA is the Numbering Plan Area, NXX is the Central Office within the NPA, and XXXX is the Station. The plan’s international calling code is 1. The NANP conforms to the International Telecommunications Union telephone numbering plan, known as E.164.
Generally, callers do not have to dial the area code if the recipient is within the same NPA. However, automatic telephony devices tend to dial the full number, area code and all.
Every area code has nearly 800 usable CO codes, or prefixes – some, like 911, are unavailable because they are dedicated to public use. Each prefix has 10,000 telephone numbers, and every area code has almost eight million telephone numbers. While this may sound like a large number, some big metros typically go through the lot in a few decades due to a combination of population growth, economic progress and increased demand for cellular and other services. Once they have been exhausted, new ones have to be issued as it is not possible to reassign existing codes.
This process of issuing new codes for jurisdictions nearing exhaustion is called ‘area code relief.’ A common solution involves splitting an NPA into two or more areas, with one retaining the existing area code while the rest get new codes. Generally, the NPA with the largest number of users gets to keep the old area code to keep disruptions to a minimum for as many people as possible.
Another common solution involves assigning an existing NPA with an additional area code, an approach known as overlaying. While this minimizes service disruptions, it does mean adjacent businesses may have different area codes, a situation that entails a little bit of extra dialing.
To ease the transition process, authorities and carriers allow for a permissive dialing window during which customers can still use the old area code as they familiarize themselves with the new one. This period typically lapses after six months, after which callers are notified if they use the old area code.
Types of Number Plan Area Codes
Fixed Length NPA Codes
Some telephone numbering plans prescribe NPA codes of an unvarying length. NANP NPA codes have a fixed three-digit length, while Australia’s telephone number plan limits NPA codes to a single digit.
Variable Length NPA Codes
A number of countries use telephone numbering plans with variable-length NPA codes. These include the United Kingdom and Germany, where area codes are anywhere from two to five digits long, and Japan, where NPA codes have a length of between one and five digits.
Easily Recognizable Codes
NANP area codes in which the second and last digits are similar are referred to as Easily Recognizable Codes (ERCs) or N11. These easy-to-remember codes, such as 411 and 911, that are used for accessing special services.
Where To Find a Complete List of NANP Area Codes
You can find a complete list of NANP area codes at the NANPA website or sites such as Area Code Locations. Most sites let you search for area codes by city and state.