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E.164 is an ITU-T recommendation, titled The international public telecommunication numbering plan, that defines a numbering plan for the world-wide public switched telephone network (PSTN) and some other data networks.

E.164 defines a general format for international telephone numbers. Plan-conforming numbers are limited to a maximum of 15 digits.[1] The presentation of numbers is usually prefixed with the character + (plus sign), indicating that the number includes the international country calling code (country code), and must typically be prefixed when dialing with the appropriate international call prefix, which is a trunk code to reach an international circuit from within the country of call origination.

The title of the original version and first revision of the E.164 standard was "Numbering Plan for the ISDN Era".

Telephone number categories[edit]

This recommendation provides the number structure and functionality for the three categories of telephone numbers used for international public telecommunication:

For each of the categories, it details the components of the numbering structure and the digit analysis required to successfully route the calls. Annex A provides additional information on the structure and function of E.164 numbers. Annex B provides information on network identification, service parameters, calling/connected line identity, dialing procedures, and addressing for Geographic-based ISDN calls. Specific E.164-based applications which differ in usage are defined in separate recommendations.

The number categories are all based on a fifteen-digit numbering space. Before 1997, only twelve digits were allowed. The definition does not include any trunk dialing codes, necessary for a call to reach international circuits from inside the country of call origination.

Number structure for geographic area[edit]

Country Code National Destination Code (optional) Subscriber Number
National (significant)
cc = 1 to 3 digits maximum = 15 − cc = 12 to 14 digits
International public telecommunication number for geographic areas (maximum 15 digits)

Number structure for global services[edit]

Country Code Global Subscriber Number
cc = 1 to 3 digits maximum = 15 − cc = 12 digits
International public telecommunication number for global services (maximum 15 digits)

Number structure for networks[edit]

Country Code Identification Code Subscriber Number
cc = 1 to 3 digits x = 1 to 4 digits maximum = 15 − (cc + x) = 8 to 11 digits
International public telecommunication number for networks (maximum 15 digits)

Number structure for groups of countries[edit]

Country Code Group Identification Code Subscriber Number
cc = 1 to 3 digits gic = 1 digit maximum = 15 − (cc + gic) = 11 digits
International public telecommunication number for groups of countries (maximum 15 digits)



This recommendation describes the procedures and criteria for the reservation, assignment, and reclamation of E.164 country codes and associated identification code (IC) assignments. The criteria and procedures are provided as a basis for the effective and efficient utilization of the available E.164 numbering resources. Such assignments require a collaborative effort between the ITU-TSB and the appropriate ITU-T study group to endeavor to ensure that the assignments meet the needs of the telecommunication community. The development of these criteria and procedures are in accordance with the principles contained in E.190 and the numbering plan formats detailed in E.164.


This recommendation contains the criteria and procedures for an applicant to be temporarily assigned a three-digit identification code within the shared E.164 country code 991 for the purpose of conducting an international non-commercial trial.


This recommendation describes the principles, criteria, and procedures for the assignment and reclamation of resources within a shared E.164 country code for groups of countries. These shared country codes will coexist with all other E.164-based country codes assigned by the ITU. The resource of the shared country code consists of a country code and a group identification code (CC + GIC) and provides the capability for a group of countries to provide telecommunication services within the group. The TSB is responsible for the assignment of the CC + GIC.


E.163 was the former ITU-T recommendation for describing telephone numbers for the public switched telephone network (PSTN). In the United States, this was formerly referred to as a directory number. E.163 was withdrawn, and some recommendations were incorporated into revision 1 of E.164 in 1997.

DNS mapping of E.164 numbers[edit]

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[2]

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. The e164.arpa domain is in production status as of 2013 only in a few European nations (Austria, Czech Republic, Finland, Germany, Ireland, Lithuania, Poland, Romania, Netherlands, United Kingdom).[3] Australia conducted a trial in 2007, but then abandoned further support of.1.6.e164.arpa.[4] Many nations have no .e164.arpa implementation active.[5]

The Internet Telephony Users Association runs e164.org as an alternate server, which relies on users submitting their own PSTN numbers and corresponding URIs for verification.[6] A VoIP PBX, such as Asterisk, may be configured to resolve numbers in both databases, sending only those not listed with a free Internet routing over the PSTN.

See also[edit]


  1. ^ ITU; ITU-T. "The International Public Telecommunication Numbering Plan". ITU. p. 11. Retrieved 21 March 2014. 
  2. ^ Geoff Huston. "ENUM —Mapping the E.164 Number Space into the DNS". The Internet Protocol Journal 5 (2). Cisco Systems. Retrieved 26 October 2013. 
  3. ^ "RIPE ENUM Working Group: Progress Matrix". Enumdata.org. Retrieved 26 October 2013. 
  4. ^ "ENUM trial". enum.com.au. Retrieved 26 October 2013. 
  5. ^ Russell Bryant, Leif Madsen, Jim Van Meggelen. Asterisk: The Definitive Guide. Retrieved 26 October 2013. 
  6. ^ Joshua Brashars (13 August 2007). Asterisk Hacking. Retrieved 26 October 2013. 

External links[edit]

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/E.164 — Please support Wikipedia.
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