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The ITU ENUM allocates a specific zone, namely e164.arpa for use with E.164 numbers. Any phone number, such as +1 555 42 42 can be transformed into a hostname by reversing the numbers, separating them with dots and adding the e164.arpa suffix, like so: 184.108.40.206.220.127.116.11.e164.arpa
An example NAPTR record looks like this :
$ORIGIN 18.104.22.168.22.214.171.124.e164.arpa. IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!sip:email@example.com!" . IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:firstname.lastname@example.org!" .
This specifies that if you want to use the "sip+E2U" service, you should use sip:email@example.com as the address. The regular expression can be used by a telephone company to easily assign addresses to all of its clients; e.g. if your number is +15554242 your SIP address is sip:firstname.lastname@example.org and if your number is +15551234 your SIP address is sip:email@example.com.
ITU e164.arpa subdomains are first delegated to ('registered by') regulatory bodies designated by the national government of the country code concerned, which further delegates zones to telecommunications providers. Your telephone company is therefore in charge of the NAPTR records, usually. Some countries are proposing to let end-users register their own telephone numbers via an intermediary which need not be their own telco. This is a good idea, since Enum is mostly useful for VoIP (Voice over IP) - people who use an Enum-enabled VoIP service can dial your existing number, and be connected not to your existing phone line, but to your own VoIP telephone, directly via the Internet, bypassing the telephone system entirely - but when they call some one who does not use Enum, calls run over the conventional telephone system.
Alternative free public ENUM services such as E164.org have also emerged. These services often verify PSTN numbers, and can be used in addition to the e164.arpa. Some perceive competitive ENUM zones as a good thing, since it will open the VoIP market to competition from the beginning.
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