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How LNP Works

Local Number Portability (LNP) is made technically feasible by the Location Routing Number (LRN), a unique 10-digit telephone number assigned to each switch. The LRN approach made it possible to introduce LNP without radically changing the Public Switched Telephone Network. It allowed the existing routing paradigm to remain in place, permitting a gradual conversion of the network to handle LNP traffic.

Before LNP was established, the NPA-NXX of a telephone number identified the state and rate center where the number was originally assigned, the service provider and the carrier type (wireline or wireless). Today, because telephone numbers have been ported between wireline and wireless service providers, the NPA-NXX of a telephone number only identifies the state and rate center where the number was originally assigned. The LRN’s NPA-NXX now serves as the network address that helps communications service providers cost-effectively route calls and traffic to their proper destinations.

Calls are routed based on the first six digits (NPA-NXX) of the telephone number. The NPA-NXX is the address of the switch serving the telephone number. When a number is ported, the 10 digit LRN is associated with the ported number. Calls to the ported number are instead routed based on the NPA-NXX of the 10 digit LRN.

Before LNP was established, the NPA-NXX of a telephone number identified the switch serving the number, the state and rate center where the number was originally assigned as well as the service provider. Because LNP allows a number to be moved from one switch to another, the NPA-NXX of a telephone number is no longer a reliable indicator of the serving switch and service provider’s identity. Today, since numbers have been ported between wireline and wireless service providers, the NPA-NXX of a telephone number no longer can be relied on to determine whether the number is served by a wireline or wireless service provider.

  Decoding the Telephone Number
NPA Number Plan Area (Area Code)
NXX Prefix ( Central Office Code)
XXXX Line Number
  • N can be any number from 2 to 9
  • P, A, can be any number from 0 to 9
  • X can be any number from 0 to 9

For example: (571) 434-5400:

  • 571 is an area code assigned in Virginia
  • 571-434 is an exchange assigned to Verizon Virginia, Inc. associated with the Herndon rate center and served from the switch HRDNVASTDS0.
  • 5400 is the line number assigned to the Neustar PBX

LNP relies on the location routing number method

The Location Routing Number (LRN) is a ten-digit number designed to look like a telephone number. An LRN is assigned to each ported telephone number and is used to route calls through the PSTN to the switch serving the ported number. The LRN is intended only as the network address of the serving switch, however, and is not intended to convey any rate area information such as would be used determine whether a call is local or long distance. The same ten-digit LRN can be used for every ported number served from the switch to which the LRN’s NPA-NXX is assigned. The LRN is not a telephone number. The same ten-digit string could be used both for an LRN and for an individual telephone number assigned to the LRN’s switch. However, the best practice is to avoid using the same ten-digit string for both a consumer’s telephone number and an LRN.

Porting a number to another carrier

There are three scenarios for why telephone numbers are ported: inter-carrier/competitive porting, intra-carrier porting and number pooling (in the U.S.).

Type of Porting What is Does Effect on Telephone Numbers
Inter-carrier or Competitive Moves the number from the carrier currently providing service to the carrier that will be providing service. Changes the switch where the number resides and the carrier providing service to the end user.
Intra-carrier Adds a record in the NPAC; may also move a number from one switch to another within the same carrier’s network. Used when a carrier that holds the number puts it in the NPAC for reasons other than competitive porting. There is no change in carrier.
Number Pooling Numbers are assigned to a new service provider in blocks of 1,000 to create an inventory of unassigned phone numbers, e.g., 571-434-5000 through 5999. These number blocks are put in the NPAC. Calls to a telephone number in the block must be routed by the block’s LRN unless the block is in the same switch as its NPA-NXX.

Steps in the LNP Porting Process

To understand the steps in the porting process, it may be helpful to explain the steps of the process. Below is a high-level summary of the processing steps and porting process for a typical competitive port. In this case, a consumer is switching to a new communications service provider and wants to keep his existing telephone number.

  1. The new service provider notifies the old service provider of the requested port.
  2. The old service provider is asked to validate the subscriber’s information.
  3. The old service provider confirms the subscriber’s information and notifies the new service provider.
  4. The new service provider notifies the NPAC of the requested port.
  5. The NPAC creates a pending port and sends a notification to the old service provider.
  6. Optionally, the old service provider notifies the NPAC that it concurs with the port.
  7. The new service provider notifies the NPAC to activate the port.
  8. The pending port is activated in the NPAC and broadcast to the telecommunications industry network within milli-seconds.

Each NPAC record, referred to as a Subscription Version, contains various pieces of information about the telephone number including:

  • The telephone number
  • The current assigned service provider ID (SPID)
  • The service provider type (such as wireless or wireline)
  • The LRN
  • SS7 Destination Point Codes ( LIDB, CNAM, CLASS, etc.)
  • Service Type (such as class 2 VoIP or pre-paid wireless)
  • Alternative SPID (to identify a reseller)
  • Billing ID
  • End user location and type

For carriers using automated solutions for processing and if there are no errors or issues with validations and notifications, the porting process is generally completed within minutes. Even providers who use manual processes, if there are no errors or issues with validations, and it is a simple port, the FCC has mandated that the request be completed within one business day.

Call Flow to a Ported Telephone Number

When a call is made to the ported telephone number, the initiating service provider switch launches a query to its LNP call routing database to determine whether the telephone number has been ported. If it has, the database response provides the switch with the LRN needed to properly route the call. If the number is not ported, the database response indicates that the call should be routed based on the telephone number. When multiple switches are involved in the call path, the next to last carrier has the responsibility to make the LNP database query if one has not already been made.


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