The NANP number format may be summarized in the notation NPA-NXX-xxxx:

Component Name Number ranges Notes
NPA Numbering Plan Area Code Allowed ranges: [2–9] for the first digit, and [0-9] 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.[19] 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: [2–9] for the first digit, and [0–9] for both the second and third digits (however, in geographic area codes the third digit of the exchange 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 [0–9] 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 central office code must not be in the form N11. (For non-geographic area codes like +1-800, 9-1-1 remains invalid, but other N-1-1 codes are valid exchange prefixes[citation needed])
  • 314-159-2653 is invalid, because the office code must begin with 2-9.
  • 123-234-5678 is invalid, because NPA cannot begin with 0 or 1
  • 281-234-5678 is valid.

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