Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematically organized by the E.164 standard, while the Internet uses the Domain Name System (DNS) for linking domain names to IP addresses and other resource information. Telephone number mapping systems provide facilities to determine applicable Internet communications servers responsible for servicing a given telephone number using DNS queries.

The most prominent facility for telephone number mapping is the E.164 number to URI mapping (ENUM) standard. It uses special DNS record types to translate a telephone number into a Uniform Resource Identifier (URI) or IP address that can be used in Internet communications.

Being able to dial telephone numbers the way customers have come to expect is considered crucial for the convergence of classic telephone service (PSTN) and Internet telephony (Voice over IP, VoIP), and for the development of new IP multimedia services. The problem of a single universal personal identifier for multiple communication services can be solved with different approaches. One simple approach is the Electronic Number Mapping System (ENUM), developed by the IETF, using existing E.164 telephone numbers, protocols and infrastructure to indirectly access different services available under a single personal identifier. ENUM also permits connecting the IP world to the telephone system in a seamless manner.

For an ENUM subscriber to be able to activate and use the ENUM service, it needs to obtain three elements from a Registrar:

This works as follows: (1) the Registrar provides the Subscriber (or Registrant) with a domain name, the URI, that will be used for accessing a DNS server to fetch a NAPTR record, (2) a personal E.164 telephone number (the ENUM number). The URI domain name of (1) is biunivocally associated (one-to-one mapped) to the subscriber E.164 ENUM number of (2). Finally (3) the NAPTR record corresponding to the subscriber URI contains the subscriber call forwarding/termination preferences.

Therefore, if a calling party being at the PSTN network dials a called party ENUM number by touch typing the E.164 called party number, the number will be translated at the ENUM gateway into the corresponding URI. This URI will be used for looking-up and fetching the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (either on IP or on PSTN terminations) – the so-called access information – which the registrant (the called party) has specified by writing his/her choice at the NAPTR record ("Naming Authority Pointer Resource Records" as defined in RFC 3403), such as e-mail addresses, a fax number, a personal website, a VoIP number, mobile telephone numbers, voice mail systems, IP-telephony addresses, web pages, GPS coordinates, call diversions or instant messaging. Alternatively, when the calling party is at the IP side, the User Agent (UA) piece of software of the dialler will allow to dial a E.164 number, but the dialler UA will convert it into a URI, to be used for looking-up at the ENUM gateway DNS and fetch the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (again, either on IP or on PSTN terminations).

Calling by using a new personal E.164 number (the ENUM number) to look-up at a database is therefore an indirect calling support service.

The ITU ENUM allocates a specific zone, namely "e164.arpa" for use with ENUM E.164 numbers on the IP side of the network. RFC 6116 defines how any E.164 number, such as +1 555 42 42 can be transformed into a URI, by reversing the numbers, separating them with dots and adding the e164.arpa suffix thus: 2.4.2.4.5.5.5.1.e164.arpa