How it works?
The Internet and telephony are two completely separate worlds, each governed by its own rules. ENUM brings these worlds together. So, in the same way that a domain name serves as the basis for e-mail, SMTP, POP, etc, a phone number can be used as a peg on which to hang a variety of services.
A phone number is ideally suited to serve as a unique identifier, to which other details can be linked. The reason being that every phone number in the world is unique and structured according to a global standard: the e164 number plan of the ITU (International Telecommunications Union). Although it isn't apparent to the user, ENUM works with phone numbers in reverse order. This is because domain names are hierarchically arranged from right to left, while phone numbers are structured the other way around. By reversing a phone number and adding ‘e164.arpa', you get a domain name that can be used on the Internet or, in fact, on any IP network. So the (fictitious) Dutch phone number +31 (0) 20 123 4567 becomes 7.6.5.4.3.2.1.0.2.1.3.e164.arpa.
Just as you don't see the numeric IP address that you browser uses when you visit a website, but a domain name instead, so the reversed phone number is hidden from view. So you don't have to start memorising phone numbers. All you do is enter a name from your address book and indicate that you want to send a message. An ENUM application invisibly reverses the number in order to search the ENUM databases for appropriate contact details (NAPTR records). The message is then delivered using the mode that the addressee has selected.
With ENUM, it is up to the phone number subscriber to specify the mode in which incoming messages are received - regardless of where the person is (at home, at work, on holiday) and what method is used to send the message (computer, phone, PDA, SMS, mobile, fax, etc). Everyone can specify his or her preferred message delivery mode using an application on a computer, PDA or phone.