Numbering System
The NANP number format, including the E.164 country calling prefix, can be summarized in the notation +1-NPA-NXX-xxxx.
| Component | Name | Number ranges | Notes |
|---|---|---|---|
| +1 | ITU country calling code | "1" is also the usual trunk code for accessing long-distance service between NANP numbers. In an intra-NANP context, numbers are usually written without the leading "+" | |
| NPA | Numbering Plan Area Code | Allowed ranges: for the first digit, and for the second and third digits. When the second and third digits of an area code are the same, that code is called an easily recognizable code (ERC). ERCs designate special services; e.g., 888 for toll-free service. The NANP is not assigning area codes with 9 as the second digit. | Covers Canada, the United States, parts of the Caribbean Sea, and some Atlantic and Pacific islands. The area code is often enclosed in parentheses. |
| NXX | Central Office (exchange) code | Allowed ranges: for the first digit, and for both the second and third digits (however, the third digit cannot be "1" if the second digit is also "1"). | Often considered part of a subscriber number. The three-digit Central Office codes are assigned to a specific CO serving its customers, but may be physically dispersed by redirection, or forwarding to mobile operators and other services. |
| xxxx | Subscriber Number | for each of the four digits. | This unique four-digit number is the subscriber number or station code. |
For example:
- 234-235-5678 is valid
- 234-911-5678 is invalid, because the exchange code cannot be in the form N11.
- 123-234-5678 is invalid, because NPA cannot begin with 0 or 1
- 291-234-5678 is invalid, because the second digit of an area code cannot be 9
The country calling code for the NANP is +1. In international format, an NANP number should be listed thus: +13015550100 (example using the original area code for Maryland). The trunk prefix code for direct-dialed long-distance in the NANP is also 1.
Each three-digit area code may contain up to 7,919,900 unique phone numbers:
- NXX may begin only with the digits, providing a base of 8 million numbers: ( 8 x 100 x 10000 ) .
- However, the last two digits of NXX cannot both be 1, to avoid confusion with the N11 codes (subtract 80,000).
- Despite the widespread usage of NXX "555" for fictional telephone numbers — see 555 (telephone number) — today, the only such numbers specifically reserved for fictional use are "555-0100" through "555-0199", with the remaining "555" numbers released for actual assignment as information numbers (subtract 100).
- Several other NXX prefixes are generally not assigned: the home area code(s), adjacent domestic area codes and overlays, area codes reserved for future relief nearby, industry testing codes (generally NXX 958 and 959) and special service codes (such as NXX 950 and 976).
Read more about this topic: Telephone Numbers In The United States
Famous quotes containing the words numbering and/or system:
“The task he undertakes
Is numbering sands and drinking oceans dry.”
—William Shakespeare (1564–1616)
“Daily life is governed by an economic system in which the production and consumption of insults tends to balance out.”
—Raoul Vaneigem (b. 1934)