link to X.121 Address

X.121 Address

X.121 Address also called an international data number (IDN), is an address of an end node (computer or terminal) that is connected to an X.25 public data network.

link to X.400


X.400 is a set of standards defined in 1984 and 1988 by the International Telecommunication Union (ITU) for computer-based handling of e-mail.

link to X.25 Protocol

X.25 Protocol

X.25 is a packet-switching protocol for wide area network (WAN) connectivity that uses a public data network (PDN) that parallels the voice network of the Public Switched Telephone Network (PSTN).