semantic numbers on the web



ENUM is the acronym for "Electronic Numbering", "Electronic Number Mapping", "E.164 to Universal Resource Identifier Mapping" and several more.  ENUM stands for a series of technical protocols and infrastructure arrangements, trying to bridge the gap between the world of Telecommunications and the Internet . ENUM enables the universal reach of a called party independently of the device or application used by means of a single unique 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 or a Web URL.

The Basics of ENUM

ENUM is a protocol that defines a method to convert a regular telephone number (e.g. +43 780 555 111 in the <SemNum> project) 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 general, the DNS provides a set of records to be used for different purposes. ENUM uses the "Naming Authority Pointer Resource Record" (NAPTR RR), which allows to store a set of URIs in the DNS being retrievable by means of a qualified DNS query. Figure 1 shows an example ENUM query for the Austrian telephone number +43 780 555 111 resulting in an answer comprising an e-mail address, a Web URL and a SIP URI. 

Figure 1: ENUM Basics

In the traditional telephone system, the most significant number appears first, e.g.  the Austrian country code (+43) in the example above. In Internet domain names, however, the most significant information appears last, e.g. the top-level domain for Austria (.at) in 

In order to bring the telephone number into an Internet compatible format, ENUM reverses the sequence of the digits of a telephone number in the international format and puts dots between each digit. With a valid top-level domain added, each digit becomes a node in the domain name hierarchy. 

Figure 2: ENUM translation procedure

In order to maintain the integrity of the telephone numbering space and its equivalent in the ENUM DNS name space, the Internet Architecture Board (IAB) has proposed the domain name “” for ENUM purposes. However, alternative ENUM trees like,, and have spread in the meantime beside this official "golden tree". In our example the phone number  +43 780 555 111 becomes in the official ENUM tree.

URIs and ENUMservices

ENUM allows to link a single E.164 telephone number with a whole range of URIs and associated services. As a Web URL can be used for retrieving Web site data or for retrieving public key information, for instance, the service associated with a given URI has to be specified. In ENUM this is done by a range of ENUMservices proposed in a technical standard (TS) published by the European Telecommunications Standardisation Institute (ETSI). Table 1 lists those ENUMservices; some of them have already passed the necessary registration process with the Internet Assigned Numbers Authority (IANA).

„ENUMservices“ for interactive media-stream exchange







„ENUMservices“ for discrete (non-session related) messages























„ENUMservices“ for information source





„ENUMservices“ for service resolution services





„ENUMservices“ for session-oriented message-exchanges





„ENUMservices“ for instant information display – announcement








„ENUMservice” for redirection





„ENUMservices“ for location information





„ENUMservices“ for public key information




Table 1: List of ENUMservices

The list of ENUMservices is open for extension if demand should arise to include additional services being developed in the future. 

User ENUM and Carrier ENUM

ENUM comes in two flavours: User ENUM on the one hand, and Carrier ENUM on the other.

User ENUM is a form of electronic business card. The phone number becomes a universal key that is globally accessible on the Web. Anyone knowing the universal key, i.e. the user's phone number, can have access to the user's data stored in ENUM. As all data is publicly available on the Web (in the DNS), privacy implications have to be taken into account before storing data in ENUM. However, the user always stays in control over the data entered in ENUM and published on the Web. In short, User ENUM provides a way to find services of other users on the Web. 
As the holder of a telephone number always has to be in control of the data associated with his/her phone number, ENUM domain names are only allowed for existing phone numbers. Therefore the ENUM domain name holder has to be the assignee of the related E.164 number. ITU-T and the national regulatory authorities are involved in the administration of the tree to make sure the integrity of E.164 telephone numbering space and ENUM domain name space are assured.

Carrier ENUM is used by carriers to determine the ingress points to other carriers' IP networks. The basic principle of Infrastructure ENUM is to provide information only to other service providers and possibly to a number of selected peers, only. Typically, the end user has no access to this Carrier ENUM information or may not be able to use it.