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Sunday, 30 September 2012

How does ENUM work?

How does ENUM work?

A telephone number, once entered, is translated into an Internet address. The following steps explain the process:
  • The number entered is translated into a full qualified E.164 number. (To quote an example from earlier, the number 9777 1234 in Melbourne, Australia would be +61 3 9777 1234.)
  • The number is reduced to digits only. (e.g., 61397771234)
  • The digits are re-ordered back to front. (e.g., 43217779316)
  • Dots are placed between each digit. (e.g.,
  • The domain e164.arpa is added to the end. (e.g.,
  • A DNS query is then issued on this domain.
Once the authoritative name server is found, ENUM retrieves relevant NAPTR Resource records and will perform according to the user's registered services for that number.

What is .arpa, and why is it the top level domain for ENUM?
Why not create a new top-level domain specifically for ENUM?

E164.arpa has been designated as the temporary DNS domain for use with ENUM. This designation may change as a result of ongoing discussions between the ITU, the IETF and other international organizations involved with ENUM. (In the event that the international community chooses a different ENUM domain, the structures discussed here and in IETF RFC 2916 will apply to that new designated domain.)
The .arpa domain has been designated for Internet infrastructure purposes. It is managed by the Internet Assigned Numbers Authority (IANA) in cooperation with the Internet technical community under the guidance of the Internet Architecture Board (IAB). A new top-level domain (e.g., .e164) was not created because ENUM is an infrastructure application appropriate for designation within the previously established .arpa domain. ENUM is considered appropriate as an infrastructure application because it provides a set of DNS-based resource directories, referenced by phone number, for use by various ENUM-enabled application clients (such as telephones, SIP servers and voice messaging systems).

Why is the number reversed?

DNS, when reading a domain name, will first search for the top level domain (.arpa), then the second level domain (being e.164). It then searches for the country code, area code and phone number.

Why are there dots between the numbers?

Each dot separates the number into administrative domains, or zones. This allows for delegation of authority at various points along the name, and eliminates the requirement for clients to know individual delegation schemes to know where to put the dots.

Will ENUM telephone routing confuse the PSTN routing system?

ENUM will not affect PSTN application level functions such as call routing and signaling. ENUM facilitates the discovery of resources associated with a telephone number, and does not impact how applications operate.
It is important to note that ENUM will not change the existing right-to-use rules and principles for telephone numbers. ENUM is intended to facilitate applications using telephone numbers as subscriber names, not to change how telephone numbers are administered.



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