What is the OSI model?
The OSI model is a conceptual framework that describes how network communication works in 7 standardised layers. Each layer performs a specific function in the process of transmitting data between devices over a network.
The OSI model helps you to better understand, structure and troubleshoot network protocols, devices and processes.
🧱 The 7 layers of the OSI model
| Layer | Name | Function |
|---|---|---|
| 7 | Application | Interface with the user (e.g. HTTP, SMTP, FTP) |
| 6 | Presentation | Handles encoding, compression and encryption (e.g. TLS, JPEG) |
| 5 | Session | Manages connections/sessions between systems (e.g. RPC, NetBIOS) |
| 4 | Transport | Reliable data transmission (e.g. TCP, UDP) |
| 3 | Network | Routing and IP addressing (e.g. IP, ICMP) |
| 2 | Data Link | Physical addressing (MAC), error detection (e.g. Ethernet, PPP) |
| 1 | Physical | Transmits bits over cables, radio or light (e.g. RJ45, Wi-Fi, RS-485) |
📦 Visualisation
7 - Application ← User ↔ network software (e.g. browser) 6 - Presentation ← Encryption, compression 5 - Session ← Manages logins and connections 4 - Transport ← Delivery reliability, segmentation 3 - Network ← IP routing 2 - Data link ← MAC addressing, frames 1 - Physical ← Signals, bits on the cable
🔍 Why is the OSI model important?
- Troubleshooting: helps you better understand where something is going wrong (e.g. network vs. application)
- Standardisation: ensures interoperability between vendors and technologies
- Structure: assists with the design, documentation and segmentation of networks
- Education and certification: foundational knowledge for CCNA, IEC 62443 and OT security
📌 In summary
The OSI model divides network communication into 7 layers, each with its own function — from the physical connection to application-level interaction. It provides insight, structure and guidance for the design, security and troubleshooting of networks.