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E.164 is an ITU-T recommendation which defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers.
The title of the original and revision 1 was "Numbering Plan for the ISDN era".
E.164 addresses can be used in DNS by using Electronic Numbering (ENUM) which allocates a specific zone, primarily e164.arpa for use with E.164 numbers. Any phone number, such as +1 555 42 42 can be transformed into a hostname by reversing the numbers, separating them with dots and adding the e164.arpa suffix, like so:
DNS can then be used to look up internet addresses for services such as SIP VoIP telephony. An alternate way that would be more resilent to single point of failure is DUNDi , which is a P2P implementation of ENUM 
E.163 was the old ITU-T standard 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 deleted and incorporated into revision 1 of E.164 in 1997.
This recommendation provides the number structure and functionality for the three categories of 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, dialling 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 as follows:
Number structure for geographic area
|Country Code||National Destination Code (optional)||Subscriber Number|
|cc=1-3 digits||maximum 15-cc digits|
|National (significant) number|
|International public telecommunication number for geographic areas (maximum 15 digits)|
Number structure for global services
|Country Code||Global Subscriber Number|
|cc=3 digits||maximum 12 digits|
|International public telecommunication number for global services (maximum 15 digits)|
Number structure for networks
|Country Code||Identification Code||Subscriber Number|
|cc=3 digits||x=1-4 digits||maximum 12-x digits|
|International public telecommunication number for networks (maximum 15 digits)|
Recommendations contained in E.164
- E.164.1, Criteria and procedures for the reservation, assignment and reclamation of E.164 country codes and associated Identification Codes (ICs)
- 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.
- E.164.2, E.164 numbering resources for trials
- 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.
- E.164.3, Principles, Criteria, and Procedures for the Assignment and Reclamation of E.164 Country Codes and Associated Identification Codes for Groups of Countries
- 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 of countries. The TSB is responsible for the assignment of the CC + GIC.
- Defense Informations Systems Agency. Commercial Telecommunications Standards: ITU-T Reccommendations.
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