How LNP Works

A guide to LNP and telephone number coding

Local Number Portability (LNP) is possible thanks to the Location Routing Number (LRN), a unique 10-digit telephone number assigned to each switch. 

Prior to the establishment of LNP, a telephone number’s original state, rate center, service provider and carrier type (wireline, wireless, or VOIP) were identified in the North American Numbering Plan (NANP) by the NPA (area code) and NXX (central office code). With the introduction of LNP, telephone numbers were being ported between service providers and a new way of determining the network address of the serving switch was needed to ensure the call was routed correctly.  With that, the LRN was created. 

How LRN Enables LNP

The LRN is a unique 10-digit telephone number assigned to each switch to support call routing through the public switched telephone network (PSTN). It serves as the network address and includes the NPA and NXX for a telephone number. A single LRN can be used for every ported number served between a switch and the NPA-NXX assigned location. The assignment of an LRN allows for LNP. 

Porting a telephone number to another carrier

There are three reasons a telephone number might need to be ported:

  • Inter-carrier or competitive porting; when a telephone number moves from the current service provider to a new service provider.
  • Intra-carrier porting; when a telephone number moves from one switch to another within the same telecommunications service provider's network.
  • Number pooling; when a telephone number moves to a new service provider in blocks of 1,000 to create an inventory of unassigned telephone numbers.

Each telephone number entered into the NPAC’s records, is called a “Subscription Version” and contains the following information:

  • The current assigned service provider ID (SPID)
  • The service provider type (wireless or wireline or VoIP)
  • The LRN
  • SS7 Destination Point Codes 
  • Service Type Alternative SPID (optional)
  • Billing ID (optional)
  • End user location and type (optional)

The entire porting process can take just minutes. The FCC has mandated simple one-number port requests be completed within one business day.

Call flow for ported telephone numbers

A call is made to a ported telephone number

  • The Initiating service provider’s switch launches a query to find out if the number has been ported
  • If yes the LRN of the new switch is provided
  • If not, the call is routed based on the telephone number


Steps of the LNP Porting Process

How LNP Works