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NPA/NXX: What Area Codes and Exchanges Mean

NPA/NXX: What Area Codes and Exchanges Mean

If you have ever looked at a phone number and wondered what the different parts mean, or if you work in telecom and need to understand how numbering data drives routing decisions, this article breaks down the structure of North American telephone numbers from the ground up.

The 10-Digit Number

Every telephone number in the North American Numbering Plan (NANP) follows the same structure:

NPA - NXX - XXXX
(312)  555 - 1234
  • NPA (Numbering Plan Area): the three-digit area code
  • NXX (Central Office / Exchange code): the three-digit exchange
  • XXXX (Subscriber / Station number): the four-digit line number

Together, these ten digits uniquely identify a telephone line (or, more precisely, a telephone number assignment) within the NANP’s coverage area: the United States, Canada, and twenty Caribbean and Pacific island nations, all sharing country code +1.

NPA: The Area Code

The NPA is the first three digits. It identifies a geographic region (or, in some cases, a service type like toll-free). When you see area code 312, you know the number is associated with Chicago, Illinois. When you see 201, that is northern New Jersey.

Format Rules

NPAs follow the format N X X, where:

  • N = any digit from 2 through 9 (the first digit is never 0 or 1)
  • X = any digit from 0 through 9

This gives a theoretical range of 800 possible NPAs (200-999). In practice, many are reserved:

  • N11 codes (211, 311, 411, 511, 611, 711, 811, 911) are reserved for special services. 911 is emergency services; 411 is directory assistance; 211 is community services, and so on. These are not area codes.
  • N9X codes (390, 590, etc.) have historically been reserved, though some are now being opened as the numbering plan faces exhaustion pressures.
  • 37X and 96X are reserved for future use.
  • Toll-free NPAs: 800, 888, 877, 866, 855, 844, 833 (and eventually 822) are non-geographic toll-free codes.
  • 900 is the premium-rate services NPA.
  • 500, 533, 544, 566, 577, 588 are reserved for personal communications services.

After all reservations, roughly 320-330 geographic NPAs are actively assigned to states and regions across North America.

Geographic vs. Non-Geographic NPAs

Most NPAs are geographic — they map to a specific region within a state or province. Area code 415 covers San Francisco. 469 covers the Dallas metropolitan area.

Some NPAs are non-geographic. Toll-free numbers (800, 888, etc.) do not correspond to any location. The same is true of 900 premium-rate numbers and 500 personal numbers. When you see one of these NPAs in routing data, the routing logic is fundamentally different — toll-free calls require a database lookup to determine the responsible carrier, rather than a simple NPA/NXX route.

Overlays

When a geographic region runs out of NXX assignments within its original area code, a new NPA is overlaid on the same territory. Chicago’s 312 shares its footprint with 773 and 872. Manhattan’s 212 shares with 332, 646, and 917. Overlay NPAs require 10-digit dialing since the area code is needed to distinguish numbers in the same local calling area.

NXX: The Exchange Code

The NXX is digits four through six. Historically, this was called the central office code or exchange code because it identified the specific telephone switch (central office) serving the subscriber. Today, with number portability, the NXX no longer reliably tells you which switch handles a call — but it remains the foundational unit for numbering administration and routing data.

Format Rules

NXX codes follow the same N X X format as NPAs: the first digit must be 2-9, and the remaining two can be 0-9. This yields 800 possible NXX codes per NPA.

A few NXX values are special:

  • 555: Traditionally reserved for directory assistance and fictional use (the famous “555” numbers in movies). In practice, only 555-1212 is universally reserved for directory assistance. The range 555-0100 through 555-0199 is reserved for fictional use. Other 555 numbers may be assigned normally.
  • 950: Used for Feature Group B access codes (dialing into specific long-distance carriers).
  • 976: Historically used for premium-rate local information services.
  • 958 and 959: Often used for telephone company test lines (though this varies by region).

What an NXX Tells You

Each NXX assignment in the NANP carries associated metadata that is critical for telecom operations:

  • Carrier (OCN): The Operating Company Number of the carrier that was assigned this NXX block. For example, OCN 9323 is AT&T Illinois.
  • Rate Center: The geographic point associated with the NXX for rating and routing purposes. Rate centers like CHICAGO or HACKENSCK determine local vs. long-distance classification and are used in calculating intercarrier compensation.
  • LATA: The Local Access and Transport Area. LATAs are geographic regions originally created during the Bell System breakup to define boundaries between local and long-distance service.
  • Type: Whether the NXX block is assigned for landline, wireless, or VoIP use.
  • Status: Whether the assignment is active, reserved, or has been returned to the pool.

You can explore this data for any area code in our area code directory. For example, the Chicago 312 area code page shows every NXX assignment with its carrier, rate center, and type.

Walking Through a Number: (312) 555-1234

Let us break down what each piece of the number (312) 555-1234 tells a telecom system:

NPA 312: This number is in the Chicago, Illinois numbering plan area. The originating carrier’s routing table knows that 312 maps to Illinois, LATA 358, timezone America/Chicago. If the caller is outside the 312/773/872 overlay complex, this is likely a long-distance call.

NXX 555: Within the 312 area code, exchange 555 is assigned to a specific carrier and rate center. The carrier’s NPA/NXX routing data shows which trunk group or SIP route to use for reaching this exchange.

Subscriber 1234: The last four digits identify the individual line. These are not visible in routing data — they are only relevant at the terminating switch, which uses them to ring the correct line or device.

In practice, before routing the call, the originating carrier also performs a Number Portability lookup. Even though the NPA/NXX data says 312-555 was originally assigned to a particular carrier, the actual subscriber at 312-555-1234 may have ported their number to a completely different carrier. The Location Routing Number (LRN) returned by the NPAC database tells the originating carrier where to actually send the call.

How NPA/NXX Data Drives Routing

For telecom carriers and VoIP operators, NPA/NXX data is operationally essential in several ways:

Rate Decks and Billing

Wholesale voice carriers publish rate decks — pricing tables that assign a per-minute rate to every NPA/NXX combination. An NXX in rural Alaska costs more to terminate than one in Manhattan because the underlying interconnection costs differ. Rate decks are built on NPA/NXX-to-rate-center mappings.

Least Cost Routing

Carriers with multiple upstream providers use NPA/NXX data to implement least cost routing (LCR). For each NPA/NXX, the routing engine compares rates across available carriers and selects the cheapest (or best quality-adjusted) route. This happens in real time on every call.

Regulatory Compliance

NPA/NXX data determines jurisdictional classification. Whether a call is local, intrastate long distance, or interstate long distance affects which regulatory rules and access charges apply. The rate center and LATA assignments associated with each NXX are the basis for these determinations.

Number Provisioning

When a carrier provisions a new phone number for a customer, they draw from their assigned NXX blocks. The NPA and rate center determine the geographic identity of the number — a business in downtown Chicago expects a 312 number, not an 847 (suburban) number.

NPA/NXX Assignment Process

NPA codes are assigned by NANPA (the North American Numbering Plan Administrator), currently operated by Somos, Inc. A state or region requests a new NPA when existing ones approach NXX exhaustion.

NXX codes within an NPA are assigned by the carrier requesting them, subject to regulatory approval. Since the introduction of thousands-block pooling, carriers can receive blocks of 1,000 numbers (a single NXX is 10,000 numbers) to reduce waste. Pooling is mandatory in most US states and has significantly extended the life of the numbering plan.

The NPA/NXX assignment data is public record and updated continuously as new codes are activated and existing ones are reassigned. Our reference data is refreshed daily from NANPA and internal carrier routing tables to reflect current assignments.

Further Reading

The North American Numbering Plan (NANP)