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:

This works as follows: (1) the Registrar provides the Subscriber (or Registrant) a domain name, the URI, that will be used to access 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 to look-up and fetch 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 2915, 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. Alternately, 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 to look-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 3761 define how any ENUM 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 to look up at the DNS the Internet addresses for services such as SIP VoIP telephony. NAPTR records are used to set the subscriber call forwarding/ termination preferences. Therefore the whole system can 'translate' E.164 addresses to SIP addresses. An example NAPTR record is:

$ORIGIN 2.4.2.4.5.5.5.1.e164.arpa.  IN NAPTR 100 10 "u" "E2U+sip"  
"!^.*$!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 sip: phoneme@example.net 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 sip:4242@555telco.example.net; if your number is +15551234, your SIP address is sip:1234@555telco.example.net.

The following Figure illustrates how ENUM works by giving an example: Subscriber A sets out to call Subscriber B.

The ENUM user does not notice anything of this reversal and the DNS database look-up, as this is done automatically behind the scene using a user agent software in his PC or terminal, or at the PABX or Gateway. For instance, when the user types the telephone number in his web browser ENUM enabled agent and indicates what item of information he is 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 his 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 he is currently unaware that he is using the DNS when he makes a connection with the Internet or what is going on at the telephone switch when he makes a call.