How does ENUM work?

Probably the easiest way of explaining how ENUM works is to give you an example. Let us suppose you want to call DENIC.  You dial the following number: +49 69 27 235 0. If your telephone terminal supports ENUM, it transcribes the number as

Next, an inquiry about is submitted to the Domain Name System (DNS).

This inquiry retrieves the transcription and resolution rules, i.e. the so-called Naming Authority Pointer (NAPTR) Resource Records. The output is a URI for accessing IP-based applications. These addresses can be reached via the Internet. One of these is now selected for use for continuing the communication. Let us assume that the DNS outputs the following:
 IN NAPTR 10 10 "U" “E2U+sip“ "!^.*$!!" .

IN NAPTR 20 10 "U" “E2U+voice:tel“ "!^.*$!tel:+4969272350!“ .
 You, the caller, want to set up a voice link through the Internet, so the system selects the NAPTR-RR
 IN NAPTR 10 10 "U" “E2U+sip“ "!^.*$!!" .
 The terminal application now sets up a communication link with the selected address through the Session Initiation Protocol.

What is a NAPTR RR?

A NAPTR RR (Naming Authority Pointer Resource Record) is an entry in the Domain Name System (DNS) containing rules for transcribing inquires. The result is returned to the application making the inquiry. For resolving ENUM, this means that an inquiry about an ENUM domain is handled according to the appropriate rules and answered with a URI (Uniform Resource Identifier). This URI provides the output address and the protocol to be used for further communication.

For example, an inquiry to the DNS about results in the return of the following NAPTR RR.

IN NAPTR 10 10 "U" "E2U+sip" "!^.*$!!" .
IN NAPTR 20 10 "U" "E2U+voice:tel" "!^.*$!tel:+4969272350!" .

Each NAPTR RR has the following format:
Class Type Order Preference Flags Service Regexp Replacement


  The preference value 10 causes the selection of the NAPTR record:
 IN NAPTR 10 10 "U" "E2U+sip" "!^.*$!!" .
 The telephone number used for the inquiry is mapped to an SIP address. The result is thus a URI. This is arrived at by interpreting the regular expression "!^.*$!!", which means that the complete inquiry string is to be replaced by the string

What is an E.164 number?

E.164 is a recommendation issued by the ITU (ITU-T Rec. E.164) for the international telephone-number plan. An E.164 number is made up of decimal digits only and must satisfy particular requirements as regards its structure, length and uniqueness. As part of the number plan, the individual countries have been assigned particular national codes (Germany, for instance, has 49). All this is described in the Annex to ITU-T Rec. E.164.

What is the meaning of

.arpa is an infrastructure domain that is used as the Top Level Domain for ENUM domains. is a Second Level Domain. The individual national codes are placed under German numbers, for instance, are mapped under

In the case of infrastructure domains, the name is not of decisive importance, since users do not need to use it directly. What is much more important is reliable supervision of the system and its stable technical operation.

Reliable system supervision has been taken care of, since the .arpa domain is accessible via all root-name servers. Independence of operation is also guaranteed, since the domain is to be looked after by RIPE NCC. That has been determined in the Agreement that came from the ITU.

What do domains referencing telephone numbers and ENUM domains have in common?

There is, in fact, no direct link between these two types of domains. For instance, is a "catchy" name for a website offering various Internet services.

The ENUM domain to which the telephone number 0800 6666666 is mapped would be This domain is clearly less suitable for entering in the address line of a browser. It could, however, be used by the application is such a way that when the telephone number 0800 6666666 is dialled a DNS inquiry establishes various services (telephone, e-mail, webpage, etc.) that are available for the particular service. Assuming the end user has an application that can show web pages, it might be possible to call the site and the connection would be established that way.

Why does ENUM use numbers instead of domains with mnemonic names?

The big achievement of the Domain Name System (DNS) is that it greatly simplifies what would otherwise be a painstaking process using unwieldy IP numbers. By assigning each service to a domain, it is possible to address it through a meaningful name. The service for assigning names is available across networks. This has been by no means the least significant factor that has contributed to making the Internet so popular for broad sections of the population.

So why then should ENUM now bring numbers back to the setting up of communication links?
 The first point to make is that that has so far been the usual way to make telephone calls! Modern VoIP communication protocols, such as the Session Initiation Protocol (SIP), do, however, use addresses that are similar to e-mail, which, in turn, are much easier to manage. If, however, the only device available for calling communication services is a simple telephone set (with the numeric keypad as its only input device) then there is no other choice than to use numeric addresses, i.e. telephone numbers. The function of ENUM is to provide a technology that enables Internet communication and conventional telephone services to be linked to one another. The use of ENUM makes it simple for a subscriber in the conventional telephone network to call another subscriber in the Internet.

Any user who happens to have a more sophisticated terminal device can also call their communication partner by means of a simple mnemonic address, such as sip:user@domain.tld.

In future, the simple way of addressing all individually allocated services via a single contact address might be through a domain with a format like firstname.surname.domain.TLD. An inquiry to the DNS would then return a NAPTR Resource Record. This would contain the user's telephone number as well as his/her other addresses for individual services. The application making the inquiry would then be able select one of these addresses and use it for the communication (telephone call, e-mail, http query and so on and so forth).

What precisely was tested during the ENUM trial at DENIC?

The initial test phase primarily involves general testing of the potential opened up by the new ENUM technology. That means developing scenarios and adding the requisite ENUM functions to the necessary software and hardware. Information on experience and findings ought to be made freely available to all trial participants and test partners throughout the field trial. DENIC's contribution includes a public mailing list and the publication of reports on already implemented testbeds.
 The Final ENUM-Field-Report is available in english.

What is meant by ENUM?

What ENUM is all about is telephone-number mapping; it is a protocol which connects resources form the telecommunications and Internet areas. It defines a rule which is used to map telephone numbers onto domains in an unambiguos way. These domains can then be used for the identification of various communication services. Examples include numbers for fax and mobile telephony, voice-mail systems, e-mail addresses, IP telephony addresses, web pages, GPS coordinates, call diversions or unified messaging. The procedure is documented in RFC3761.

What telephone numbers are eligible for registration as ENUM domains at DENIC?

The only numbers that can be registered as ENUM domains at DENIC are those that belong to the German national telephone-number space (+49).

During the initial trial phase, the following telephone numbers are permitted as ENUM domains and as entries in the NAPTR record: 

  • Geographic numbers: area code + subscriber number with a maximum of 15 digits inkcl. national-number
  • Mobile telephone numbers belonging to the bands: (0)15 + subscriber number, (0)16 + subscriber number, (0)17 + subscriber number
  • National free-phone numbers: (0)800 + subscriber number
  • Personal numbers: (0)700 + subscriber number
  • VoIP-Numbers: (0)32 + subscriber number

DENIC is envisaging adding other telephone numbers as the trial develops and is also planning to clarify other framework conditions.

What possibilities does ENUM offer the end user?

ENUM creates a whole series of new communication possibilities: 

  • Conventional terminal equipment with only twelve keys (0-9, * and #) can make use of Internet services.
  • Telephone calls via Interent free of charge: The connection between two "traditional" telephone systems with IP connection is established via the Internet, if the telephone system of the caller is suited for ENUM and the dialled number is ENUM-registered. So no phone call charges are incurred, which can mean tremendous cost savings particularly for large companies or organizations.
  •  Re-direction of SMS and faxes to e-mail boxes: This may be an additional feature provided by the telephone network. But Internet programs are also able to interpret ENUM entries directly and thus send a scanned-in fax image, for example, immediately to the desired e-mail address.
  • The NAPTR entries help to set priorities in ENUM with regard to the communication line to be chosen. Example: First call the Internet phone; if you cannot reach me there, try the fixed-line service; if that attempt fails, call my mobile. But you can also choose all available terminal devices to be addressed simultaneously, i.e. the Internet telephone as well as the fixed-line end device and the mobile phone will all ring, when the corresponding number is called.
  • Any network elements and services in the Internet can be found using one single telephone number.

For whom is the ENUM operation intended?

ENUM is intended for anyone interested in gathering experience with the new technology as soon as possible and in building up their know-how.

This includes, for instance, the following: 

  • Internet Service Providers who want to extend their systems for the registration of ENUM domains;
  • Service Providers who want to use the advent of the new technology to build up new services for their clients;
  • Developers of software and hardware applications who want to integrate ENUM support in their applications;
  • Research departments and research institutions interested in bridge-building between the two worlds of the public-service telephone network (PSTN) and the Internet;
  • End users with interest in technology who would like to build up their own first-hand experience with this particular new technology at an early stage of its development.

What can I do if false or imcomplete data are stored for my ENUM domain?

If you yourself are the holder of the ENUM domain concerned, you ought to contact your provider and tell them the correct data, so that it can be communicated to DENIC.

How often does a reload of the name servers take place?

Before a newly registered ENUM-domain becomes accessible round the world it has to be included in the zone. This is usually done the next time the name servers are reloaded following completion of the registration. The same applies to modifications to name server entries for ENUM-domains already registered. The following are the approximate times for the name server reloads:

 Reloading the zone: between 1:00 and 2:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 1:00 CET.
 Reloading the zone: between 4:00 and 5:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 4:00 CET.
 Reloading the zone: between 7:00 and 8:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 7:00 CET.
 Reloading the zone: between 10:00 and 11:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 10:00 CET.
 Reloading the zone: between 13:00 and 14:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 13:00 CET.
 Reloading the zone: between 16:00 and 17:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 16:00 CET.
 Reloading the zone: between 19:00 and 20:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 19:00 CET.
 Reloading the zone: between 22:00 and 23:00 CET
 This zone contains all the domain jobs that DENIC confirmed before 22:00 CET.

Which nameservers are authoritative for the zone?

The following name servers are authoritative for the zone zone:

Folgende Nameserver sind für die zuständig (autoritativ):


Which nameservers are authoritative for the zone?
Host nameIP addressLocation

What is a Top Level Domain?

The Top Level Domains (TLDs) are the highest hierarchical level in the international Domain Name System (DNS). They are found at the end of the Domain, to the right of the last dot. We make a distinction between general or generic TLDs (gTLDs), such as .com, .net and .org, and country code TLDs (ccTLDs), such as .de (Germany) or .ch (Switzerland).

Domains for some of the gTLDs can be registered by anybody, whereas for others, such as .gov, .int, .aero or .museum, registration is reserved for particular user groups (in the four cases listed: the US government, international organisations, companies and institutions concerned with aviation, and museums). The coordination of the gTLDs is in the hands of the international organization ICANN.

The individual ccTLDs are administered by so-called Network Information Centers (NICs). For Germany, that is DENIC. A list of all the existing domain endings and the registries that administer them can be found on IANA's website.