Ask Mr. Andrews: What is ENUM?

August 12, 2008 by Garrett Smith

Dear Mr. Andrews,

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

ENUM is a protocol developed by the IETF ( 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
Internet Infrastructure domain defined for this purpose (

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: (Each dot separates the number into administrative domains, or zones.)
  5. The domain “” is appended to the end. Example: (.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.


  • Karl Fife

    Should we care about it? As far as I know there is still no way for individuals or even good sized business to populate the tree–public enum trial notwithstanding. This project is getting old and stale. After all these years it is still providing nothing of use. Am I wrong?

  • Cory Andrews

    Karl – I agree, it’s moved at a snail’s pace…but I am of the opinion that you will see renewed interest in the near term and in the business space around providing ENUM services. ENUM is one piece of an overall larger pie that has been in the oven for a long time as you know.

  • Karl Fife

    ENUM is only as good as the tree we can all agree upon and trust, so I hope you’re right. It seems that there is a greater shortage of bureaucratic decisiveness and efficiency than interest per se. I wonder how much the incumbents are dragging their feet because they are not economically served by the success of ENUM, nor the larger journey toward network agnosticism for real-time communication.

    I already perceive a lot of interest–enough to spawn great projects like, DUNDI and ISN. These technologies try and accomplish the same or similar end result. Do you have any reason to believe that we are any closer to the IETF’s official real ENUM tree, or are you making the point that there will be even more ENUM-LIKE projects that will be borne of the brave new world of telecom growing tired of waiting for the ‘real’ ENUM.

  • Cory Andrews

    Karl – Neustar ( seems to be the main body behind ENUM, and they cetainly have their own commercial interests.

    Tough to tell whether a private sector, commercial enterprise or an open, not-for-profit collaboration will be the one to bring the concept to fruition.

    Do you have any insights regarding the relationship between Neustar and the FCC?

  • dear sir we like to know how we can get your service for calling by enum as you say in this
    page taht we can create phone number by our dns or by our web site or email so this is good
    we like to know how we can get this .

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