ENUM is a scheme described in RFC 6116 for mapping E.164 telephone numbers to SIP addresses, XMPP addresses and various other types of resource. ENUM can enable more rapid discovery of services, network resilience and convenience for people who only have a telephone number or a numeric dialpad.

Telephone numbers are everywhere. People have large collections of phone numbers in their mobile telephone address books. Many organizations have large databases of customer phone numbers that have either been submitted by customers or collected from caller-ID. In public ENUM, the owner of a number is able to give callers specific options for contacting them. In private ENUM, organizations can distribute a rich set of routing data in a highly efficient and convenient format, DNS.

ENUM must be considered from two perspectives: publishing your own data for ENUM users and making use of the ENUM data published by other people and organizations.

For many people, the first impression of ENUM is the public ENUM tree. Given that many countries have not yet formally adopted ENUM, the public ENUM tree still has some big gaps and is not universally useful for every telephone number that exists in the PSTN. Fortunately, the story does not end there. There are several unofficial ENUM trees and there are many ways that ENUM can be used within an organization.

ENUM is worth looking at for various reasons, including the widespread support in many open source products, the relative simplicity of using distributed and fault-tolerant DNS servers to serve such data and the ease of troubleshooing using basic DNS tools.