The OSI Model
Understand the 7-layer OSI model and how each layer plays a role in network communication.
What is the OSI Model?
The Open Systems Interconnection (OSI) Model is a conceptual framework that divides network communication into 7 distinct layers. It helps engineers understand, design, and troubleshoot networks by providing a common language.
Memory tip: "Please Do Not Throw Sausage Pizza Away" — Physical, Data Link, Network, Transport, Session, Presentation, Application.
The 7 Layers (Top to Bottom)
Layer 7 — Application
The layer you interact with directly. Provides network services to end-user applications. Protocols: HTTP, HTTPS, FTP, DNS, SMTP.
Layer 6 — Presentation
Translates data between the application and the network. Handles encryption, compression, and data formatting. Example: SSL/TLS encryption.
Layer 5 — Session
Manages sessions (connections) between applications. Establishes, maintains, and terminates communication sessions. Example: NetBIOS, RPC.
Layer 4 — Transport
Provides reliable data transfer between endpoints. Handles segmentation, flow control, and error correction. Protocols: TCP (reliable) and UDP (fast, no guarantees).
Layer 3 — Network
Handles routing of packets across different networks using IP addresses. Routers operate at this layer. Protocol: IP (IPv4 / IPv6).
Layer 2 — Data Link
Handles node-to-node data transfer within the same network segment. Uses MAC addresses. Switches operate here. Protocol: Ethernet, Wi-Fi (802.11).
Layer 1 — Physical
The actual hardware — cables, fibre optic, radio waves. Transmits raw bits (0s and 1s). Examples: RJ45 cables, Ethernet ports, Wi-Fi antennas.
Why Does It Matter?
The OSI model helps isolate problems. If a website won't load, you can check layer by layer: Is the cable plugged in (Layer 1)? Is the IP configured (Layer 3)? Is the web server running (Layer 7)?
What's Next?
Learn how the internet practically implements these concepts with TCP/IP, or explore HTTP & HTTPS for Layer 7 in action.