X.121 is a hierarchical addressing scheme that was originally designed to number X.25 nodes. X.121 addresses are up to 14 digits in length and are structured as follows:
The first digit is a zone number that identifies a part of the world. For example, Zone 2 covers Europe and Zone 3 includes North America). The zone numbers can be found in Appendix C, "List of ITU-TX.121 Data Country or Geographical Codes." These codes can also be found in ITU-T Recommendation X.121.
- Service Provider: 1 digit
- Terminal Number: Up to 10 digits
• E.164 is a hierarchical global telecommunications numbering plan, similar to the North American Number Plan (NANP). E.164 addresses are up to 15 digits in length and are structured as follows:
This code is based on the international telephony numbering plan and can be found in Appendix D, "International Country Codes." These codes can also be found in any phone book.
• National Destination Code and Subscriber Number: Up to 14 digits in length (maximum length is dependent on the length of the Country Code).
Subaddress: Up to 40 digits
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