X.25 is a Communication protocol standardized by packet switching in point-to-point mode offering of many services.

X.25 integrates the three low layers of the OSI model ( Open Systems Interconnection ) which comprises 7 of them:

• level 1: physical Layer
• level 2: layer connection
• level 3: layer network
• level 3 1/2: Lay down impermeable network, in version LAPB of H.Fernandez

This standard was established in 1976 by CCITT (then begun again by UIT-T) for the networks with packet switching on proposal of 4 countries which use it for their public networks of communication: TRANSPAC for France, EPSS for Great Britain, Datapac for Canada, DCS for Belgium and Telenet for the USA.

X.25 defines the interface between a ETTD (Final Equipment of Data processing) and a ETCD (Data circuit terminating equipment) for the transmission of packages. It thus fixes the operating rules between a user of the network and the network itself.

X.25 is a growing old technology which tends to disappear. X.25 is still used in networks such as the Réseau of the aerial navigation and in much of banking institutions to manage to them GAB.