Card 05/ 09
The OSI Model vs the TCP/IP Model
Seven layers to reason with, and the four layers the internet is actually built on.
The OSI model and the TCP/IP model both describe network communication as a stack of layers, each built on the one below. The OSI model has seven layers and is a reference model, designed to describe any network. The TCP/IP model has four layers and describes the protocols the internet actually runs on, which were designed before the OSI model was finished.
The two differ on three things worth comparing: how many layers there are, what they are used for, and where the session and presentation jobs go.
OSI modela reference model, for describing and reasoning
Seven layers: Physical, Data Link, Network, Transport, Session, Presentation, Application. Each has a separate job, including keeping a conversation together (session) and agreeing a data format (presentation). Engineers use its numbers as a shared vocabulary: layer 2, layer 4, layer 7.
TCP/IP modelthe protocol suite the internet runs on
Four layers: Link, Internet, Transport, Application. Session and format are the application's own business, so HTTP, TLS and JSON all live in the one top layer. Its layers are named after the protocols in them: IP is the internet layer, TCP and UDP the transport.
| OSI layers | TCP/IP layer | Example protocols |
|---|---|---|
| 7 Application, 6 Presentation, 5 Session | Application | HTTP, DNS, TLS, SSH |
| 4 Transport | Transport | TCP, UDP |
| 3 Network | Internet | IP, ICMP |
| 2 Data Link, 1 Physical | Link | Ethernet, Wi-Fi |
The session and presentation layers are where the models part. In practice no widely used internet protocol sits at layer 5 or 6 alone: an HTTP client opens and closes its own connections, and character encoding is a header inside the HTTP message. That is why the TCP/IP model folds them into the application.
The rule for choosing. Use the OSI numbers when you are saying where a fault or a device sits, because "layer 4" and "layer 7" are what every vendor and colleague uses. Use the TCP/IP layers when you are reading how a real protocol is built, because they match the specifications. Most engineers use both in the same conversation, and the mapping above is all it takes to move between them.
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