OSI & TCP/IP Model

The OSI model splits networking into seven stacked layers, from the application at the top down to the physical wire at the bottom. The whole point is *separation of concerns*: each layer has one job and talks only to the layer directly above and below it — so a web browser never has to know whether it's running over fibre or Wi-Fi.

The seven layers, top to bottom

1. 17 · Application — the protocols apps speak: HTTP, DNS, SMTP. This is the data you actually care about.
2. 26 · Presentation — formatting, translation, encryption, compression: how the data is represented (e.g. TLS, character encoding).
3. 35 · Session — opening, managing, and closing a conversation between two endpoints.
4. 44 · Transport — end-to-end delivery, split into segments, addressed by ports. TCP adds reliability; UDP doesn't.
5. 53 · Network — logical addressing (IP) and routing a packet across many networks. Routers live here.
6. 62 · Data Link — node-to-node delivery on one physical link, using MAC addresses and framing. Switches live here.
7. 71 · Physical — raw bits as electrical, optical, or radio signals on the medium.

Encapsulation: nested envelopes

Sending data walks *down* the stack. At each layer a header of control information is wrapped around whatever came from above — this is encapsulation. Receiving walks *up* the stack and strips those headers back off in reverse order (decapsulation). Crucially, each layer's header is read only by its counterpart on the other machine: the receiver's Network layer reads the sender's Network header, and so on.

The data chunk has a different name at each layer — its PDU (protocol data unit): it's Data up at Application through Session, a Segment at Transport, a Packet at Network, a Frame at Data Link, and finally raw Bits at Physical.