What is NANP and why does it define US phone number format?

The North American Numbering Plan (NANP) is the telephone numbering plan shared by the US, Canada, and 20+ other territories. It divides every 10-digit number into a 3-digit area code (NPA), a 3-digit exchange code (NXX), and a 4-digit subscriber number. The key structural rule is that the first digit of both NPA and NXX must be 2–9, because 0 and 1 are reserved as signalling prefixes by the public switched telephone network.

Why do area codes that start with 0 or 1 fail?

NANP reserves leading 0 for international access and leading 1 as the North American country code. Any area code beginning with those digits would be misrouted by telephone switches, so the ITU and NANP have never assigned them as geographic codes. The validator flags these immediately.

What are N11 codes and why are they rejected?

N11 codes (211, 311, 411, 511, 611, 711, 811, 911) are special abbreviated dialling codes assigned to specific services — social services, non-emergency police, directory assistance, and so on. They occupy the same NPA position as a geographic area code but are never assigned as ordinary telephone area codes, so any number using one of those three digits as its area code is structurally invalid as a regular phone number.

What is the 555 exchange and should I be concerned?

The 555 exchange (NXX) has a special status in NANP. Within 555, the range 555-0100 through 555-0199 is permanently reserved as a "fictitious number" block for use in film, TV, and testing — these numbers will never be assigned to real subscribers. Numbers outside that range within 555 may or may not be assigned; some (like 555-1212 for directory assistance) are real. The validator flags 555 exchanges and specifically labels the 0100–0199 range as unassigned fiction numbers.

Does this tool confirm the number is active or reachable?

No. Structural validation only confirms the number fits NANP rules — correct digit count, valid area code, valid exchange. It cannot confirm whether the number has been assigned to a subscriber, whether it is currently active, or whether it has been ported to a different carrier. For that level of confirmation you would need a live lookup service (HLR lookup or a carrier API), which involves a network call and typically a cost per query.

What is E.164 format and when should I use it?

E.164 is the ITU international telephone numbering standard. For a US number it prepends the country code 1 directly to the 10-digit NANP number with a plus sign, giving +1NPANXXXXXX — for example +12125550142. E.164 is the correct format for SIP URIs, SMS APIs (Twilio, AWS SNS, Vonage), REST payloads, and international dialling strings. The domestic (NPA) NXX-XXXX format is fine for display in US-only UIs, but E.164 is the safe default for any system that may route internationally or integrate with a telephony API.

US Phone Number Validator

A US phone number validator that checks every digit of your number against the North American Numbering Plan (NANP) — the official framework that governs how telephone numbers are structured in the United States (and 25 other NANP territories). Paste in a number in any common format and the tool breaks it down into its three components, explains any rule that fails, and delivers two normalised outputs ready to copy: the standard US display format and the international E.164 string.

How it works

NANP divides every US telephone number into exactly 10 digits in three groups:

NPA (Numbering Plan Area) — the 3-digit area code, such as 212 for Manhattan or 415 for San Francisco.
NXX (Exchange code) — the next 3 digits, originally the central-office exchange identifier.
XXXX (Subscriber number) — the final 4 digits assigned to the individual line.

The validator strips every non-digit character from your input (spaces, dashes, parentheses, dots) and then strips a leading 1 if you included the country code, leaving a bare 10-digit string. It then enforces four rules:

Length — exactly 10 digits (or 11 with a leading 1).
NPA first digit — must be 2–9; 0 is the international access prefix and 1 is the NANP country code, so neither can open an area code.
NPA not an N11 code — 211 through 911 (where the second digit is 1 and the third is also 1) are reserved for abbreviated services and are never assigned as geographic area codes.
NXX first digit — must be 2–9 for the same reason as the NPA.

The tool also recognises the 555 exchange and specifically labels numbers in the 555-0100 through 555-0199 range as the permanently unassigned “fictitious number” block that film and television productions use to avoid involving real subscribers.

Worked example

Suppose you receive the string +1 (212) 555-0142 from a web form.

Strip non-digits: 12125550142
Strip leading country code 1: 2125550142
NPA: 212 — first digit 2, valid; not an N11 code; not reserved.
NXX: 555 — first digit 5, valid; but this is the 555 exchange.
Subscriber: 0142 — numeric value 142, which falls in 100–199.
Result: structurally valid — 555-01xx unassigned fiction range.
Formatted: (212) 555-0142
E.164: +12125550142

Now try 911-555-0100:

NPA: 911 — this is an N11 service code.
Result: invalid — N11 codes are not geographic area codes.

Input	NPA	NXX	Result
(212) 555-0142	212	555	Valid (fiction range)
415-867-5309	415	867	Valid
1-800-555-1234	800	555	Valid (800 toll-free)
911-555-0100	911	555	Invalid — N11 code
012-345-6789	012	345	Invalid — NPA starts with 0

Every check runs locally in your browser — nothing is sent to any server.