Open Systems Interconnection: the standard mental map for every layer of networking, top to bottom.

Data physically only exists as electrical/light/radio signals — the OSI model is the human-friendly abstraction layered on top so engineers can reason about "what's happening" without tracking every electron. Each layer talks logically to its counterpart layer on the other machine, but physically only ever talks to the layer directly above/below it on the same machine.

A classic mnemonic, top-down: All People Seem To Need Data Processing — Application, Presentation, Session, Transport, Network, Data Link, Physical.

Think of two people (computers) wanting to talk. On your side, data goes down the stack (A→P→S→T→N→D2→Physical), gets physically transmitted, and on the friend's side it goes back up the stack in reverse. Each layer adds its own header as data descends (this is called encapsulation), and each corresponding layer strips its header back off as data ascends on the receiving side (decapsulation).

App ↓ Presentation ↓ Session ↓ Transport ↓ Network ↓ Data Link ↓ Physical

Physical ↑ Data Link ↑ Network ↑ Transport ↑ Session ↑ Presentation ↑ App (friend receives)