Local Exchange Routing Guide
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The Local Exchange Routing Guide, commonly known as "The LERG", is a database of NPA/NXX published every month by Telcordia Technologies (http://www.telcordia.com). The LERG is primarily used by interexchange carriers in North American Numbering Plan World Zone 1 to provide information for routing interLATA telephone calls over the Public Switching Telephone Network (PSTN), as well as telephone calls originating and terminating in NPA/NXX controlled by different Operating Companies (OCNs).
NPA/NXX refers to the first 6 digits of a telephone number in World Zone 1. For instance, for the number "1 212 867 5309", The NPA/NXX is "212/867". The LERG enables an InterExchange Carrier to identify what local exchange carrier or Operating Company that NPA/NXX belongs to, and which tandem switch that NPA/NXX is connected to.
This is particularly important as the number of local exchange carriers has skyrocketed in recent years with new regulations allowing companies that don't own their own physical last mile connection to the end user to become a CLEC.