(Electronic NUMbering) A protocol from the IETF for converting a telephone number to an IP address and vice versa so that it can be resolved by the Internet's DNS system like traditional Web site domains. Primarily designed to strengthen voice over IP (VoIP) service, ENUM is expected to provide a one-stop directory for contacting people and services and to integrate the public telephone network (PSTN) with the Internet. See DNS, VoIP and VPIM.

The E164 ARPA Domain
ENUM transforms a phone, fax or pager number into a dotted Internet address that is stored in the DNS system. For example, the fully qualified telephone number 1-215-555-1234 would turn into 4.3.2.1.5.5.5.5.1.2.1.e164.arpa. The digits are reversed because DNS reads right to left (the top level domain such as .com in a URL is read first).

Typing in a telephone number would resolve into a URL where the target could be a traditional phone, a VoIP phone or a Web service. ENUM records can hold attributes about a device such as whether it can receive a text message or is Internet capable. See E.164, RDF and XDI.