ENUM, as defined by the IETF, is a convenient and powerful way to map telephone numbers to the latest communications technologies such as SIP, XMPP (Jabber) and web sites.
ENUM resolution operates much like reverse DNS lookup:
- Take the number you want to call and normalize it into E.164 format - in other words, by removing any leading zero and punctuation and adding the country code. For example, for a British number "020 7123 4567", the country code is 44 and so the normalized form of the number is +442071234567
- Remove the leading plus symbol, reverse the digits and insert a dot between each digit. For our sample number +442071234567, it becomes 220.127.116.11.18.104.22.168.0.2.4.4
- Choose the ENUM suffix. The official ENUM tree defined by the IETF uses the e164.arpa ENUM suffix. There are several third-party suffixes as well, such as e164.org and for the sip5060.net service, use the suffix e164-addr.sip5060.net. For our example, the full string, with suffix, is 22.214.171.124.126.96.36.199.0.2.4.4.e164.arpa
- Perform a DNS lookup, typically looking for NAPTR records
Here is an example using dig. The subject number we are looking up is 01865 332244 from the Nominet contact page: