OSI Model Cheatsheet
The OSI (Open Systems Interconnection) Model is a seven-layer framework for standardizing network communication.
1. Application Layer (Layer 7)
- Purpose: Provides network services directly to user applications.
- Functions:
- Enables interaction between software applications and the network.
- Handles protocols for user interfaces.
- Examples:
- HTTP, HTTPS (Web Browsing)
- FTP, SFTP (File Transfers)
- SMTP, IMAP, POP3 (Email)
- SNMP (Network Management)
- Telnet, SSH (Remote Access)
2. Presentation Layer (Layer 6)
- Purpose: Ensures data is in a readable format.
- Functions:
- Data translation (e.g., ASCII to EBCDIC).
- Data encryption and decryption.
- Data compression and decompression.
- Examples:
- SSL/TLS (Encryption)
- JPEG, PNG (Image Compression Formats)
3. Session Layer (Layer 5)
- Purpose: Manages communication sessions between applications.
- Functions:
- Session establishment, maintenance, and termination.
- Synchronization (e.g., checkpoints in a data stream).
- Examples:
- NetBIOS
- RPC (Remote Procedure Call)
4. Transport Layer (Layer 4)
- Purpose: Ensures reliable or fast delivery of data.
- Functions:
- End-to-end communication.
- Error detection and correction.
- Flow control and segmentation.
- Protocols:
- TCP (Reliable, connection-oriented)
- UDP (Fast, connectionless)
5. Network Layer (Layer 3)
- Purpose: Determines data paths and logical addressing.
- Functions:
- Routing and forwarding of packets.
- Logical addressing (IP addresses).
- Packet fragmentation and reassembly.
- Protocols:
- IPv4, IPv6 (Logical Addressing)
- ICMP (Diagnostics)
- RIP, OSPF, BGP (Routing Protocols)
6. Data Link Layer (Layer 2)
- Purpose: Manages node-to-node communication over the physical medium.
- Functions:
- Framing of data packets.
- Error detection and correction (not recovery).
- Physical addressing (MAC).
- Examples:
- Ethernet
- Wi-Fi (802.11)
- PPP (Point-to-Point Protocol)
- VLAN Tagging (802.1Q)
7. Physical Layer (Layer 1)
- Purpose: Transfers raw binary data over physical media.
- Functions:
- Transmission of bits (1s and 0s).
- Defines hardware specifications.
- Determines data encoding and signaling.
- Examples:
- Cables (Ethernet, Fiber Optic, Coaxial)
- Connectors (RJ45)
- Radio Frequencies (Wi-Fi, Bluetooth)
- Hubs, Repeaters
OSI Model Summary Table
| Layer | Key Function | Examples |
|---|---|---|
| Application (L7) | User interface, application | HTTP, FTP, SMTP |
| Presentation (L6) | Data formatting and encryption | SSL/TLS, JPEG, PNG |
| Session (L5) | Session management | RPC, NetBIOS |
| Transport (L4) | Reliable delivery | TCP, UDP |
| Network (L3) | Logical addressing, routing | IPv4, IPv6, OSPF, BGP |
| Data Link (L2) | MAC addressing, framing | Ethernet, Wi-Fi, VLAN |
| Physical (L1) | Signal transmission | Cables, RF, Connectors |
OSI Model Visualization
[ Layer 7: Application (User Interaction) ]
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[ Layer 6: Presentation (Formatting, Encryption) ]
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[ Layer 5: Session (Session Management) ]
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[ Layer 4: Transport (TCP/UDP) ]
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[ Layer 3: Network (Routing, IP) ]
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[ Layer 2: Data Link (MAC, Framing) ]
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[ Layer 1: Physical (Signals, Media) ]