ENUM (Telephone Number Mapping, E.164 Number to URI Mapping) is an addressing protocol that converts telephone numbers to URI format (name@domain). This allows you to access a SIP, H.323 or other Internet phone user by dialing a phone number.
The ENUM function aims to ensure that users can be accessed anywhere in the world with the same number, best quality and the cheapest way. ENUM maps a phone number to an Internet address in the DNS system. Thus, a user with an ENUM number can broadcast the DNS record to which the call will be routed. Even different routes can be defined for different types of calls (fax, video, etc.).
It is possible to obtain an ENUM record as if it were a domain name. Nowadays, you can obtain this registration free of charge through many registration services and VoIP service providers.
ENUM allows normal phone (E.164) numbers to be displayed as DNS names ending in e164.arpa. A number can be decoded for one or more predefined services.
For example, a telephone number + 90-312-555-1234 will be displayed as 220.127.116.11.18.104.22.168.22.214.171.124.e164.arpa after issuing rules defined in RFC 3761 and below:
- All characters except digits are removed. (“+90-312-555-1234” becomes “903125551234″)
- A period (“.”) Is placed between each number. (“126.96.36.199.188.8.131.52.184.108.40.206”)
- The order of the numbers is reversed. (“220.127.116.11.18.104.22.168.22.214.171.124”)
- .e164.arpa is added to the end of the array. (“126.96.36.199.188.8.131.52.184.108.40.206.arp a”)
To respond to this syntax, the DNS server must have a record that looks like this:
$ ORIGIN 220.127.116.11.18.104.22.168.22.214.171.124.barley.
NAPTR 10 100 "u" "E2U + sip" "! ^. * $! Sip: email@example.com!" .
NAPTR 10 101 & quot; u & quot; & quot; E2U + h323 & quot; .
NAPTR 10 102 "u" "E2U + msg" "! ^. * $! Mailto: firstname.lastname@example.org!" .
In this record you see three different routing sequences for the address 126.96.36.199.188.8.131.52.184.108.40.206. The first is SIP, the second is H.323 and the third is the SMTP response. Device selects which service to communicate by using these records.
How It Works?
The operating principle of ENUM is similar to the DNS queries we use on the Internet. DNS NAPTR resource records are used in queries.
- The phone calls an E.164 number (+90-312-555-1234)
- Gateway translates it (220.127.116.11.18.104.22.168.22.214.171.124.e164.arpa) and asks the DNS server.
- The DNS server responds to this query with a URI (sip: email@example.com).
- The gateway sends the call to the SIP server as a SIP URI call.
- The SIP server rings the IP phone registered with the URI.