ENUM Domains

ENUM – Technical Details

The ENUM protocol was developed by the ENUM Working Group of the IETF (Internet Engineering Task Force) and documented in the Internet standard RFC3761. There are some other Internet standards for future RFCs in this field. Access to the ENUM services on the Internet's existing Domain Name System (DNS) is via a subdomain reserved for ENUM. The IAB (Internet Architecture Board ) proposed .e164.arpa for this purpose, since E.164 is the standard in which the ITU (International Telecommunications Union) describes the international telecommunications number plan and .arpa is a specific Top Level Domain used as "Address and Routing Parameter Area" for infrastructure services. It is RIPE NCC in Amsterdam that acts as the international registry for .e164.arpa. Under this subdomain, an ENUM domain is then formed according to the following rule: the digits from the telephone number are placed before the .e164.arpa ending in reverse order and separated from one another by dots. To give an example, DENIC's telephone number is +49 69 27 2350 and it is linked to the domain

As far as the German part of the ENUM name tree is concerned (i.e. .9.4.e164.arpa), it was agreed with the relevant decision-making bodies to delegate it to DENIC. The lower levels can then be administered by the providers or by the customers themselves.

Requesting an ENUM

Technically, an ENUM request functions like this: once the user has entered the desired call number, the ENUM client accesses the so-called Naming Authority Pointer (NAPTR) records via the .e164.arpa name server on the ENUM name server. These records contain information about the available services that are combined with a particular E.164 call number. The result of this ENUM request may be just one or several URIs (uniform resource identifier), including the sequence in which they are to be processed. These URIs are now used to address resources or services that are associated with the E.164 call number.

  1. Input of the fax number (+49 1234) of the subscriber at the sending device.
  2. Request of the sending device to the ENUM DNS server using the ENUM domain ( of the dialled number.
  3. Answer of the ENUM DNS server delivers a list of possible communication items, among them the information to which URL (ifax:fax@denic.de) an incoming fax message has to be sent.
  4. Establishing a connection via the Internet between the sending and the receiving fax device.