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
This section gives a simplified explanation of each OSI layer and its function in the network stack. Each layer builds on the one below it, and together they define how data moves from one device to another.
Layer 1 – Physical
Responsible for the actual transmission of bits over a medium.
Covers physical components like voltage levels, timing, connectors, and media types. Defines how 0s and 1s are converted into signals and sent.
Layer 2 – Data Link
Ensures that data is correctly delivered between two devices on the same network.
Handles addressing at the hardware level and controls access to the transmission medium. Detects and may correct errors that occurred at the physical layer.
Layer 3 – Network
Enables communication between devices across different networks.
Provides logical addressing and determines the best route for data. Handles packet forwarding and fragmentation.
Layer 4 – Transport
Manages reliable or fast delivery of data across a connection.
Splits data into segments and reassembles it on arrival. Handles flow control and error correction for end-to-end communication.
Layer 5 – Session
Establishes and maintains sessions between devices.
Coordinates when communication starts, how long it lasts, and how it is terminated. Useful when multiple streams or long-lasting connections are needed.
Layer 6 – Presentation
Transforms data into formats the receiving system can understand.
Handles translation between data types, encryption, and compression. Ensures consistency in how data is interpreted across different systems.
Layer 7 – Application
Provides services that allow software to interact with the network.
Manages how user-facing programs access and use network resources. Defines network operations at the highest level, including authentication and data exchange.
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) |