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An introduction
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112 in Europe - interview

The role of OSS
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Milliseconds are
vital - interview

ENUM - The bridge between telephony and Internet


Richard Stastny


Hans Wallner
Telekom Austria


ENUM is a common name for a series of technical protocols and infrastructure arrangements, which fill the gap between the telephone system and the Internet. ENUM is designed to enable global reach of a called party on different electronic communications devices and applications by means of just one identifier – the plain old telephone number.

ENUM is a method to convert traditional telephone numbers into a format that can be used to store and retrieve Internet addressing information, for instance an email address. With ENUM and VoIP (Voice over Internet Protocol) technology, an increasing amount of voice communications can be carried over the Internet instead of over the incumbent telephone network.

VoIP is a method of digitising voice, encapsulating the digitised voice into packets, and transmitting those packets over a packet switched IP network. For the signalling of VoIP calls, several protocols are available; SIP (Session Initiation Protocol) and H.323 are the most popular. This technology has the potential to allow users to save money and to give them more flexibility in their communications by providing additional capabilities such as instant messaging, video, presence and location based services.

How ENUM works

In general, ENUM is a protocol that defines a method to convert a regular telephone number (e.g. for Eurescom +49 6221 989 123) into a format that can be used on the Internet within the Domain Name System (DNS) to look up Internet addressing information such as Uniform Resource Identifiers (URIs). In the regular telephone system, the most significant number appears first, for example the country code +49 for Germany. In Internet domain names, however, the most significant information appears last – for example The country information “de” is last, but will be the first resolved to find the top-level domain (TLD) for Germany.

Figure 1: ENUM in a nutshell

To arrange the E164 number in an “ENUM format”, the protocol reverses the sequence of the digits in an international E.164 telephone number and puts dots between each digit so that each digit becomes a node in the domain name hierarchy. Maintaining perfect coherence between E.164 numbers and their equivalent ENUM names implies that there can be only one official ENUM name space – often called the “Golden Tree”. The Internet Architecture Board has proposed the domain name “” for this purpose.

Thus, the ENUM format of Eurescom +49 6221 989 123 would be “”.

Figure 2: How ENUM works

The second feature of ENUM is that the Internet addressing Information referred to an ENUM number is stored within the domain name system (DNS), providing routing information to reach the device with the associated ENUM number.

A third feature of the ENUM protocol is that more than one contact information can be stored in the DNS record that is belonging to a specific ENUM number. An ENUM record associated with Eurescom might contain instructions for a VoIP call (e.g. or, a facsimile call (e.g., e-mail communications (e.g. Additional services can be developed in the future to be included in the ENUM name records.

This facility would allow that the phone number in ENUM will be the single contact number for multiple contact methods for any type of communication (voice, fax, e-mail, mobile, text messaging, location based services, web pages).

What ENUM is not

ENUM does not replace the numeric Internet protocol address (IPv4 or IPv6) that will be used to interact within the IP protocol directly. ENUM has also no role in the conversion of signalling messages and media streams.

ENUM is rather a framework for mapping and processing addresses of different network types. Fundamentally, what ENUM does is to provide another way to determine the desired destination to be used to initiate a communication over the next generation network.

Although ENUM will help facilitate VoIP calls, it is important to understand that VoIP phone calls do not require ENUM, and such calls can be made wholly without ENUM by using their defined Internet addressing scheme (e.g. sip: type Address-of-Records).


The User ENUM relies on the technology that is defined in RFC 3761. ENUM assumes a business model, where the ENUM function is provided independently and optionally by ENUM service providers – a form of electronic business card concept or basic buddy list. The phone number becomes a universal key that is globally accessible by all potential correspondents of the user. The data of User ENUM are public. Anyone knowing the universal key, meaning the phone number, can have access to the information, which may have privacy implications. Even if the user subscribes to several applications and services, the user would remain the only party that has the complete control over the set of identifiers.

ENUM is optional not only for the called user, it is also optional for the calling user: the calling user may use ENUM to establish a communication with the called user, but he may also use the conventional method by establishing the call via the PSTN.

This implies that ENUM “domain names” can only be allowed for existing phone numbers and that the domain name holder can only be the assignee of the related E.164 number. Administrative control, meaning proper identification and validation, is necessary to achieve this goal and keep the E.164 numbering plan intact. This is also the reason why the ITU-T and the national regulatory authorities are involved in the administration of the tree.

In summary, User ENUM provides end-users on the Internet (see figure 2) and also end-users on the PSTN (see figure 3) a possibility to find services of other end users on the public Internet.

Figure 3: Call from the PSTN to IP

Infrastructure ENUM

User ENUM is a capability provided for end users and is optional both for the calling and for the called user. If network operators want to use IP-based technology within their networks, they cannot rely on an optional technology used by end users. Instead, they need an independent routing mechanism to find the ingress points to their networks. These network operators are also called IP communications service providers.

If DNS and ENUM technology, as described in RFC3761, is used for this purpose, this is called Infrastructure ENUM. Other terms used are Carrier or Operator ENUM.

The basic principle of Infrastructure ENUM is to provide information only to IP communication service providers, some providers may even want to provide this information only to selected peers. The end user has either no access to this information, or he may not be able to use it. This purpose is incompatible with the opt-in principle, because it needs the full population of the information at least for the number range in question. Hence, it must be implemented as an independent system.

If every IP communications service provider is only providing data for numbers hosted by the operator himself, a later merging of trees should not be a problem. If, on the other hand, providers are entering data for numbers they are not hosting themselves (e.g. data for numbers where they provide transit services), then a later merging of trees will cause problems. Therefore, it is not recommended to use Infrastructure ENUM for providing transit information.

Infrastructure ENUM technology may also be used to provide access to national number portability information stored currently in IN databases. The problem with this information is that it has only national significance, for example national routing numbers. This kind of data can therefore not be used directly in supranational Infrastructure ENUM implementations.


User ENUM is an application on the public Internet to provide connectivity directly to end users with E.164 numbers. Infrastructure ENUM is a routing mechanism to provide IP connectivity between IP-based networks of originating and destination networks of IP communication service providers in addition or as replacement to the PSTN connectivity.

Both systems may be implemented independently.


  • P. Faltstrom, M. Mealling: RFC 3761 “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)”, Date: April 2004

  • ETSI TISPAN WG: TS 101 172: Minimum requirements for interoperability of ENUM Implementations, V2.0.2 (2004-06)

  • ETSI TISPAN WG: Draft TS 101 055 (rev2, 2004-07) Infrastructure ENUM, Work in progress

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