Ask Mr. Andrews: What is ENUM?

Dear Mr. Andrews,

What is ENUM, and why should I care about it?

ENUM is a protocol developed by the IETF (www.ietf.org) that uses the Internet DNS system to translate traditional telephone numbers into IP addressing schemes.

From the IETF ENUM Charter:

The ENUM working group has defined a DNS-based architecture and
protocol [RFC 3761] by which an E.164 number, as defined in ITU Recommendation
E.164, can be expressed as a Fully Qualified Domain Name in a specific
Internet Infrastructure domain defined for this purpose (e164.arpa).

ENUM allows for the lookup a dialed telephone number in DNS to see if there are alternate ways to set up the call instead of routing the call through the PSTN. ENUM may contain a reference to a SIP URL, a telephone number to dial, a web page or an e-mail address.
Once a telephone number is entered, it is translated into an Internet address in the following manner:

1. The phone number is translated into a fully qualified E.164 number by adding the city (or area) and country code. Example: 545-1927 dialed in Washington, DC becomes +1-202-545-1927, where the “1” represents the North American country code. The “+” indicates that the number is a fully qualified E.164 number.
2. All characters are removed except for the digits. Example: 12025551234
3. The order of the digits is reversed. Example: 43215552021 (As DNS names are structured from right to left)
4. Dots are placed between each digit. Example: 4.3.2.1.5.5.5.2.0.2.1 (Each dot separates the number into administrative domains, or zones.)
5. The domain “e164.arpa” is appended to the end. Example: 4.3.2.1.5.5.5.2.0.2.1.e164.arpa (.arpa is the top level domain (TLD) used for ENUM)

ENUM answers the question of how network elements can locate services on the Internet using only a telephone number, and how telephones, with their limited input mechanism of twelve keys on a keypad, can be used to access Internet services. ENUM at its most basic is the convergence of PSTN and IP networks; it is the mapping of a telephone number from the public switched telephone network to Internet functionalities.

ENUM enables Internet-based users to make a selection from a range of services available for communicating with another person when the caller knows only a telephone number or has access only to a telephone keypad. ENUM allows users to access Internet-based services and resources from Internet-aware telephones, ordinary telephones connected to Internet gateways or proxy services, and other Internet-connected devices where input is limited to numeric digits.

ENUM enables users to specify their preferences for receiving incoming communications, and gives greater user control over communications. For example, a user can specify a preference for voice mail messages over live calls during certain times of day, or may indicate a destination for call forwarding.