OSI model
termStands for: Open Systems Interconnection Basic Reference Model (ISO/IEC 7498-1, ITU-T X.200)
The seven-layer reference model of network communication, from the physical signal at layer 1 to the application at layer 7.
A standard, not folklore: ISO/IEC 7498-1:1994 and ITU-T X.200, both still in force. Its protocols lost to TCP/IP, but its numbering became the industry's vocabulary for saying where a problem lives.
The internet's own architecture never had seven layers. RFC 1122, the host requirements of 1989, lists four, application, transport, internet and link, and says its application layer 'essentially combines the functions of the top two layers' of the OSI model, presentation and application. Session has no protocol of its own in the internet suite to point at.
What survived is the map. 'Layer 2' means the link and its frames, 'Layer 3' the IP packet and routing, 'Layer 4' TCP and UDP and their ports, 'Layer 7' the application; the numbers let a network team and an application team agree in one sentence on which half of a problem is whose. The map bends wherever one device does several jobs, a load balancer terminating TLS or a tunnel carrying Layer 2 inside UDP, which is why engineers use the layers as a vocabulary rather than a law.
Also known as: osi, osi reference model, seven-layer model, iso/iec 7498-1, x.200