X.25 is an ITU-T standard protocol suite for packet switched wide area network (WAN) communication. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections or ISDN connections as physical links. X.25 is a family of protocols that was popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. X.25 was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976.

While X.25 has been, to a large extent, replaced by less complex protocols, especially the Internet protocol (IP), the service is still used and available in niche and legacy applications.

Read more about X.25:  History, Architecture, Addressing and Virtual Circuits, Billing, X.25 Packet Types, X.25 Details