the OSI model
/ OH-ess-eye /
The OSI model is the classic seven-layer textbook map of how a network is organized. OSI stands for Open Systems Interconnection, a reference model published by the ISO standards body around 1984. It was meant to be a universal blueprint for building networks; in practice it became the teaching reference — the shared vocabulary everyone uses to point at 'which part of the network are we talking about?'
Its seven layers, from the wire upward, are: 1 physical (raw bits on a medium), 2 data link (framing and local delivery across one hop, using MAC addresses), 3 network (routing packets across many hops, using IP-style addresses), 4 transport (end-to-end delivery between processes, with optional reliability), 5 session (managing dialogues between applications), 6 presentation (data formats, encoding, encryption), and 7 application (the actual programs people use). A common mnemonic for layers 1 to 7 is 'Please Do Not Throw Sausage Pizza Away.' Each layer offers a service to the one above and uses the one below — the same layering and encapsulation ideas, just with seven named slots.
Be honest about its status: the real Internet does not run OSI. The Internet runs the TCP/IP model, which collapses OSI's top three layers (session, presentation, application) into a single application layer, giving five layers in all. So why learn OSI? Because its seven names are the lingua franca — engineers say 'a layer-2 switch' (data link) or 'a layer-7 load balancer' (application) and everyone knows what they mean. Treat OSI as a precise teaching vocabulary, not as a description of how the Internet is actually built.
When someone says 'that's a layer-3 problem,' they mean the network layer (routing/IP) in OSI terms — the seven numbers are a shared shorthand even among people who run TCP/IP, not OSI.
Seven named layers — a teaching vocabulary, not what the Internet actually runs.
OSI is a reference and teaching model, not the deployed Internet architecture. The Internet runs the five-layer TCP/IP model; OSI's session and presentation layers have no separate counterpart there.