E.164 is an international numbering plan for public telephone systems in which each assigned number contains a country code (CC), a national destination code (NDC), and a subscriber number (SN). There can be up to 15 digits in an E.164 number. The E.164 plan was originally developed by the International Telecommunication Union (ITU).
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With E.164, each address is unique worldwide. With up to 15 digits possible in a number, there are 100 trillion possible E.164 phone numbers, more than 10,000 for every human being on earth. This makes it possible, in theory, to direct-dial from any conventional phone set to any other conventional phone set in the world by inputting no more than 15 single digits.
The ITU and the Internet Engineering Task Force (IETF) are currently working on a new plan called ENUM that will expand E.164 to encompass both traditional analog phones and digital devices, including computers and other devices on the Internet. All types of communications devices -- including analog phone sets and fax machines, digital phone sets and fax machines, wireless (cellular) phone sets, pagers, digital modems, digital video terminals, and VoIP devices -- will have unique E.164 addresses with direct dialing possible from any device to any other.