OSI Model
OSI Model
The OSI model explains how data travels through a network. It splits this process into **seven layers**. Each layer has its own job — from moving raw bits to showing a website.
Understanding the OSI model helps break down complex systems, and shows where tools, attacks, or failures happen.
OSI Layer Overview
The OSI model goes from Layer 1 (bottom) to Layer 7 (top). Each layer depends on the functionality of the ones above and below it. Knowing what happens at each layer helps in analyzing protocols, spotting weaknesses, and understanding where specific tools operate.
| Layer | Number | What it does | Common Examples |
|---|---|---|---|
| Application | 7 | Interfaces with user applications and defines network services | HTTP, HTTPS, DNS, FTP, SMTP, IMAP, SSH, Telnet |
| Presentation | 6 | Translates, encrypts, and compresses data for the application layer | TLS/SSL, ASCII, JPEG, MPEG, GZIP, Base64 |
| Session | 5 | Manages and maintains sessions between systems | NetBIOS, RPC, SMB, PPTP, SOCKS |
| Transport | 4 | Provides reliable or fast delivery of data streams | TCP, UDP, SCTP, QUIC |
| Network | 3 | Handles routing and logical addressing across networks | IP, ICMP, IGMP, IPsec, BGP, OSPF |
| Data Link | 2 | Transfers frames between directly connected devices using MAC addressing | Ethernet, ARP, PPP, VLAN (802.1Q), LLDP |
| Physical | 1 | Moves electrical or optical signals across physical media | Ethernet (cabling), Wi-Fi (802.11), fiber optics, hubs, radio signals |
Easy Breakdown by Layer
Layer 1 – Physical
Moves raw data (0s and 1s) over wires or radio. Involves: cables, Wi-Fi signals, connectors.
Layer 2 – Data Link
Sends frames between devices on the same network. Uses: MAC addresses, switches, ARP.
Layer 3 – Network
Chooses the best path for data across networks. Uses: IP addresses, routers, ICMP.
Layer 4 – Transport
Breaks data into smaller parts and reassembles it. TCP ensures reliability, UDP focuses on speed.
Layer 5 – Session
Keeps a connection open and manages how long systems talk. Used for: remote access, file sharing.
Layer 6 – Presentation
Changes how data looks — like converting text or encrypting. Handles: encoding, compression, TLS.
Layer 7 – Application
Where programs like browsers and email clients connect to the network. Talks via: HTTP, FTP, DNS, SMTP.
What You Can Do with It
Understanding the OSI layers helps you:
- Know where a tool or attack operates
- Analyze traffic at the right level
- Identify weak points or misconfigurations
- Work smarter with packet captures, scans, or payloads
Protocols by Layer
| Layer | Number | Function Summary | Common Protocols |
|---|---|---|---|
| Application | 7 | Interfaces with apps like browsers and mail | HTTP, FTP, DNS, SMTP |
| Presentation | 6 | Formats, encrypts, and compresses data | TLS/SSL, Base64, JPEG |
| Session | 5 | Manages connections and sessions | NetBIOS, PPTP, SMB |
| Transport | 4 | Ensures reliable or fast delivery | TCP, UDP |
| Network | 3 | Routes packets between networks | IP, ICMP, IPsec |
| Data Link | 2 | Sends frames on the local network | Ethernet, ARP |
| Physical | 1 | Transfers raw bits via cables or air | Wi-Fi (802.11), Ethernet (cabling) |