A ten-digit number that identifies the switch serving a ported telephone number, used in North America to route calls correctly after number portability.
When a number is ported from one carrier to another, it keeps its original digits but is now served by a different switch. To route calls correctly, the network looks up the dialed number in the number portability database and retrieves its LRN, which points to the switch that currently serves it.
The lookup is usually performed by the carrier immediately before the call reaches the terminating network, often called the N-1 carrier. The LRN is then carried in the signaling alongside the original dialed number so the terminating switch can complete the call. If the dip is skipped or the data is stale, calls to ported numbers may be misrouted, delayed, or billed at the wrong rate.
LRN data is also useful beyond routing. It tells you which carrier currently serves a number, which helps with rating calls correctly, detecting traffic pumping to specific exchanges, and understanding the real destination mix of a customer's traffic. It also helps verify whether a number a customer claims to hold is actually served by the carrier they say it is.
Why it matters to carriers
Wholesale and long-distance carriers depend on LRN dips for accurate routing and cost control, and errors can lead to misrouted calls or unexpected termination charges. Using LRN data in your analytics also improves fraud detection by revealing when traffic concentrates on particular carriers or exchanges.