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
Each layer in the OSI model has a specific role. Together, they describe how data moves across a network — from your device to another system, step by step.
Layer 1 – Physical
Moves raw bits (0s and 1s) over cables or air.
Involves physical elements like Ethernet cables, fiber optics, and Wi-Fi signals.
Example: When you plug in a network cable or connect to Wi-Fi, Layer 1 is active.
Layer 2 – Data Link
Connects devices on the same local network.
Uses MAC addresses to make sure data reaches the right device.
Example: Your router sends a frame to your laptop using its MAC address.
Layer 3 – Network
Routes data between networks.
Uses IP addresses to find the best path for packets.
Example: Sending a ping to a website shows Layer 3 in action.
Layer 4 – Transport
Ensures complete delivery of data.
Splits information into segments and reassembles them correctly.
Example: TCP guarantees a webpage loads fully, UDP is used for faster but less reliable connections like video calls.
Layer 5 – Session
Manages the start, duration, and end of communication sessions.
Keeps track of active connections between systems.
Example: Maintaining a stable connection while using SSH or video conferencing.
Layer 6 – Presentation
Makes data readable and secure.
Handles file formats, encryption, and compression.
Example: TLS secures your connection when using HTTPS, and JPEG ensures images display correctly.
Layer 7 – Application
Connects software to the network.
Provides access for apps like browsers, email clients, and file transfers.
Example: Opening a website (HTTP), sending an email (SMTP), or resolving a domain name (DNS).
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) |