Editing Telephone number mapping (section)

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

*A personal Uniform Resource Identifier (URI) to be used on the IP part of the network, as explained below.
*One E.164 regular personal telephone number associated with the personal URI, to be used on the PSTN part of the network.
*Authority to write their call forwarding/termination preferences in the [[NAPTR]] record accessible via the personal URI.

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 {{IETF 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 [[International Telecommunication Union|ITU]] ENUM allocates a specific zone, namely "e164.arpa" for use with ENUM E.164 numbers on the IP side of the network. {{IETF 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

The URI can then be used for obtaining the Internet Protocol addresses for services such as the [[Session Initiation Protocol]] (SIP) VoIP telephony. In the DNS, NAPTR records are used for setting the subscriber call forwarding/termination preferences. Therefore, the whole system can 'translate' E.164 addresses to SIP addresses. An example NAPTR record is:

{{sxhl|2=zone|
$ORIGIN 2.4.2.4.5.5.5.1.e164.arpa.
IN NAPTR 100 10 "u" "E2U+sip"  "!^.*$!sip:phoneme@example.net!" .
IN NAPTR 102 10 "u" "E2U+mailto" "!^.*$!mailto:myemail@example.com!" .
}}

This example specifies that if you want to use the "E2U+sip" service, you should use <nowiki>sip:phoneme@example.net</nowiki> as the address. The [[regular expression]] can be used by a telephone company to easily assign addresses to all of its clients. For example, if your number is +15554242, your SIP address is <nowiki>sip:4242@555telco.example.net;</nowiki> if your number is +15551234, your SIP address is <nowiki>sip:1234@555telco.example.net</nowiki>.

The following Figure illustrates how ENUM works by giving an example: Subscriber A sets out to call Subscriber B. 
[[Image:Ejemplo ENUM.jpg|480px|thumb|Application scenario for Voice over IP (VoIP) with ENUM.]]

* The user agent of an ENUM-enabled subscriber terminal device, or a PBX, or a gateway, translates the request for the number +34 98 765 4321 in accordance with the rule described in RFC 6116  into the ENUM domain 1.2.3.4.5.6.7.8.9.4.3.e164.arpa.
* A request is sent to the DNS for the NAPTR record of the domain name 1.2.3.4.5.6.7.8.9.4.3.e164.arpa.
* The query returns a result set of NAPTR records, as per {{IETF RFC|3403}}. In the example above, the response is an address that can be reached in the Internet using the VoIP protocol SIP per {{IETF RFC|3261}}.
* The terminal application now sets up a communication link, and the call is routed via the Internet.

The ENUM user does not notice anything of this reversal and the DNS database look-up, as this is done automatically behind the scenes using a user agent software in their PC or terminal, or at the PABX or Gateway. For instance, when the user types the telephone number in their web browser ENUM enabled agent and indicates what item of information they are looking for (email address, telephone number, web address, etc.) in the PC or terminal the number is converted to a domain name. This is sent to ENUM servers on the Internet, which send back the NAPTR records associated with the name. The access information and any priority indicated for them are stored in these. The user gets the requested address back on their PC or terminal. ENUM therefore in fact functions as a mechanism for translating a telephone number into a domain name with the requested address or number associated with it, but without the user viewing how this is done, just as they are currently unaware that they are using the DNS when they make a connection with the Internet or what is going on at the telephone switch when they make a call.