Name the seven layers of the OSI model and give one protocol or device for each where possible. How do they map onto the TCP/IP model that the Internet actually uses?
Show hints
From bottom to top: Physical, Data Link, Network, Transport, Session, Presentation, Application.
Show the explanation
Model answer.
OSI model (bottom to top)
| # | Layer | Job | Examples | Data unit |
|---|---|---|---|---|
| 1 | Physical | Transmit raw bits over a medium | Cables, radio, hubs, repeaters | Bits |
| 2 | Data Link | Node-to-node delivery on one link, MAC addressing, error detection | Ethernet, Wi-Fi (802.11), switches | Frames |
| 3 | Network | Logical addressing and routing across networks | IP, ICMP, routers | Packets |
| 4 | Transport | End-to-end delivery between processes, ports, reliability | TCP, UDP | Segments (TCP) / datagrams (UDP) |
| 5 | Session | Establish and manage dialogues between applications | Session checkpoints (rarely a separate protocol today) | Data |
| 6 | Presentation | Data format, encoding, compression, encryption | Character encoding, data serialisation | Data |
| 7 | Application | Network services used by applications | HTTP, DNS, SMTP, FTP | Data |
TCP/IP model (commonly taught with four layers):
- Link (network access) ≈ OSI 1 + 2
- Internet ≈ OSI 3 (IP)
- Transport ≈ OSI 4 (TCP, UDP)
- Application ≈ OSI 5 + 6 + 7
Some textbooks present a five-layer hybrid that keeps Physical and Data Link separate.
Key differences
- OSI is a reference model designed before its protocols and is mainly used for teaching and troubleshooting vocabulary ("it's a layer-2 issue").
- TCP/IP was described around protocols that already worked, and it is what the Internet runs on.
- Real protocols do not always fit neatly: TLS, for example, sits between TCP and HTTP and is variously described as session, presentation or application layer.
Encapsulation: each layer wraps the data from above with its own header (Ethernet frame ⊃ IP packet ⊃ TCP segment ⊃ HTTP data); the receiver strips them in reverse.
Common follow-up: "At which layer do a switch and a router work?" A traditional switch works at layer 2 using MAC addresses; a router works at layer 3 using IP addresses. Multilayer (L3) switches also perform routing.
Original UniCircle question. Last reviewed 28 Oct 2026.