DNS mapping of E.164 numbers[edit]
Main article: Telephone number mapping
Some national telephone bodies or telephone companies have implemented an Internet-based database for their numbering spaces. E.164 numbers may be used in the Domain Name System (DNS) of the Internet in which the second-level domain e164.arpa has been reserved for telephone number mapping (ENUM). In the system, any phone number may be mapped into a domain name using a reverse sequence of subdomains for each digit. For example, the telephone number +19995550123 translates to the domain name 3.2.1.0.5.5.5.9.9.9.1.e164.arpa.[3]
When a number is mapped, a DNS query may be used to locate the service facilities on the Internet that accept and process telephone calls to the owner of record of the number, using, for example, the Session Initiation Protocol (SIP), a call-signaling VoIP protocol whose SIP addresses are similar in format (user@domain...) to e-mail addresses.
This allows a direct, end-to-end Internet connection without passing to the public switched telephone network (and back) and without incurring PSTN tolls.
As this is effectively a free call, there is little incentive for carriers to promote e164 DNS service..
ENUM - The bridge between the switched telephony network and the Internet
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ENUM (E.164 Number to URI Mapping) translates telephone numbers into Internet addresses. You can dial a telephone number and reach a SIP, H.323 or any other Internet Telephony user. This all happens in the background; you do need to do anything special while calling someone.
A server with ENUM support will lookup a dialled telephone number in the ENUM tree of the DNS to see if there's alternate ways to set up the call instead of just calling out on the PSTN telephone line. ENUM may contain a reference to a SIP URI, a telephone number to dial, a web page or an e-mail address.
ENUM is already supported by SIP Proxies like SER, Kamailio, OpenSIPS or SNOM 4S, VoIP gateways like Asterisk, Swyx, and SIP phones (SNOM).
Enum uses DNS NAPTR resource records.
ENUM RFC 6116 is a protocol developed by the IETF that uses the Internet DNS system to translate E.164 (i.e. ordinary) telephone numbers into IP addressing schemes (like SIP, H323 or Email). To register new Enumservices (or update existing ones) with IANA, RFC 6117 contains all the information you need.
