📚
OSI Model Cheat Sheet
The 7 layers of the OSI model with protocols, devices, and PDUs
🔽 Top-Down (Layer 7 → 1)
All People Seem To Need Data Processing
🔼 Bottom-Up (Layer 1 → 7)
Please Do Not Throw Sausage Pizza Away
| Layer | Name | Function | Protocols | Devices | PDU |
|---|---|---|---|---|---|
| 7 | Application | User interface and application services | HTTP, HTTPS, FTP, SMTP, DNS, SSH, Telnet, SNMP | Firewall (L7), Load Balancer, Proxy | Data |
| 6 | Presentation | Data translation, encryption, compression | SSL/TLS, JPEG, GIF, MPEG, ASCII, EBCDIC | Gateway | Data |
| 5 | Session | Establishes, manages, and terminates sessions | NetBIOS, RPC, PPTP, SIP, NFS | Gateway | Data |
| 4 | Transport | End-to-end connections, reliability, flow control | TCP, UDP, SCTP | Firewall (L4), Load Balancer | Segment (TCP) / Datagram (UDP) |
| 3 | Network | Logical addressing and routing between networks | IP, ICMP, IGMP, IPsec, OSPF, BGP, RIP | Router, Layer 3 Switch | Packet |
| 2 | Data Link | Physical addressing (MAC), error detection, frame delivery | Ethernet, Wi-Fi (802.11), PPP, HDLC, Frame Relay, ARP | Switch, Bridge, NIC | Frame |
| 1 | Physical | Physical transmission of raw bits over media | Ethernet (physical), USB, Bluetooth, DSL, ISDN | Hub, Repeater, Modem, Cables, NIC (physical) | Bits |
📊 Layer Visualization
7
Application
PDU: Data
6
Presentation
PDU: Data
5
Session
PDU: Data
4
Transport
PDU: Segment (TCP) / Datagram (UDP)
3
Network
PDU: Packet
2
Data Link
PDU: Frame
1
Physical
PDU: Bits
🎯 Exam Tips
- • Know the PDU for each layer (Bits, Frames, Packets, Segments, Data)
- • Layer 4 (Transport) handles port numbers - TCP/UDP live here
- • MAC addresses = Layer 2, IP addresses = Layer 3
- • Switches = L2, Routers = L3, Firewalls = Multiple layers