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OSI Model Cheat Sheet by

The 7-layer OSI model in one clear, exam-ready reference: each layer's name, function, key protocols, and devices — plus how data flows down the stack from application to physical. A must-have for Network+ and CCNA candidates. More free study resources at bytebarhq.com.

The 7 Layers

7 — Applic­ation
User-f­acing services. Protocols: HTTP, FTP, SMTP, DNS, DHCP
6 — Presen­tation
Data format, encryp­tion, compre­ssion. Protocols: TLS/SSL, JPEG, ASCII
5 — Session
Connec­tions between apps. Protocols: NetBIOS, RPC, PPTP
4 — Transport
End-to-end delivery, ports. Protocols: TCP, UDP. Devices: Firewall (stateful)
3 — Network
Routing, logical addres­sing. Protocols: IP, ICMP, OSPF, BGP. Devices: Router, L3 switch
2 — Data Link
Switch­-to­-sw­itch, MAC addres­sing. Protocols: Ethernet, ARP, PPP. Devices: Switch, bridge, NIC
1 — Physical
Bits on the wire. Devices: Hub, repeater, cables
All 7 layers, what lives at each, and the mnemonics that make them stick.

Mnemonics

Top → bottom
All People Seem To Need Data Processing
Bottom → top
Please Do Not Throw Sausage Pizza Away
Say them out loud until they stick.

Data Names per Layer

4 Transport
Segment (TCP) / Datagram (UDP)
3 Network
Packet
2 Data Link
Frame
1 Physical
Bits
Exam trick: What PDU at Layer 3? → Packet. Layer 2 → Frame. Layer 4 → Segment.

TCP/IP Model vs OSI

Applic­ation
7 + 6 + 5 (Appli­cation, Presen­tation, Session)
Transport
4 (Trans­port)
Internet
3 (Network)
Network Access
2 + 1 (Data Link, Physical)
The 4-layer model collapses OSI's top three and bottom two.

Quick Exam Hits

ARP
Works at Layer 2/3 boundary (resolves IP → MAC)
TLS
Lives at Layer 6 (Prese­nta­tion)
Switches vs Routers
Switches = Layer 2 (MAC addresses) · Routers = Layer 3 (IP addresses)
MTU
MTU issues and fragme­ntation → Layer 3
If you remember nothing else, remember these four.

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  • Published: 14th September, 2026
  • Last Updated: 20th September, 2026

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