The seven layers, top to bottom
| # | Layer | PDU | Function · examples |
|---|---|---|---|
| 7 | Application | Data | User-facing protocols — HTTP, FTP, DNS, SMTP |
| 6 | Presentation | Data | Encoding, encryption, compression — TLS/SSL, JPEG, ASCII |
| 5 | Session | Data | Establishes/maintains sessions — RPC, NetBIOS |
| 4 | Transport | Segment | End-to-end delivery, ports — TCP, UDP |
| 3 | Network | Packet | Logical addressing, routing — IP, ICMP; routers |
| 2 | Data Link | Frame | MAC addressing, switching — Ethernet, ARP; switches |
| 1 | Physical | Bits | Signals on the medium — cables, hubs, NIC |
How to memorize the order
- Top → bottom (7→1): All People Seem To Need Data Processing.
- Bottom → top (1→7): Please Do Not Throw Sausage Pizza Away.
Layer 8?
Half-jokingly, network engineers call the user "Layer 8" — most faults trace back there.
How it maps to the TCP/IP model
| TCP/IP model | OSI layers it covers |
|---|---|
| Application | 7 Application + 6 Presentation + 5 Session |
| Transport | 4 Transport |
| Internet | 3 Network |
| Network Access / Link | 2 Data Link + 1 Physical |
Exam traps
- Switch vs router layer. Switches = Layer 2 (MAC); routers = Layer 3 (IP). (L3 switches blur this.)
- TLS layer. Encryption is usually placed at Layer 6 (Presentation).
- Segment vs packet vs frame. Transport = segment, Network = packet, Data Link = frame.
Test yourself
4 quick questions — tap an answer to check it instantly. Nothing is sent anywhere.
1. A router primarily operates at which OSI layer?
Answer: B. Routers use logical (IP) addressing at Layer 3. Switches operate at Layer 2.
2. The PDU (data unit) at the Transport layer is a…
Answer: C. Transport = segment, Network = packet, Data Link = frame, Physical = bits.
3. Which layer handles MAC addressing and switching?
Answer: B. MAC addresses and switches live at Layer 2, the Data Link layer.
4. TCP and UDP operate at which layer?
Answer: B. TCP and UDP are Transport-layer (Layer 4) protocols.
Sources
- ISO/IEC 7498-1, Information technology — Open Systems Interconnection — Basic Reference Model.
- CompTIA Network+ and Cisco CCNA exam objectives.
Reference summary only. For the formal standard see ISO/IEC 7498-1.