OSI Model (Open Systems Interconnection)

The OSI (Open Systems Interconnection) model is a conceptual framework that splits network communication into seven layers, each with a defined job and serving only the layer above it. It is a teaching and troubleshooting reference rather than a protocol that devices implement directly.

Why it matters

Layers give network engineers a shared vocabulary and a systematic way to isolate faults, usually from the bottom up:

1. Physical: cables, radio, voltage and light signals.
2. Data link: frames and MAC addresses on one segment (Ethernet, Wi-Fi).
3. Network: IP addressing and routing between networks.
4. Transport: end-to-end delivery, ports (TCP, UDP).
5. Session: managing dialogs between applications.
6. Presentation: encoding, compression, encryption formats.
7. Application: protocols the user's software speaks, such as HTTP and DNS.

Devices are often described by layer: a switch is layer 2, a router layer 3, a "layer 7 load balancer" understands HTTP.

Example

A page will not load. Cable plugged in and link light on (layer 1). Gateway reachable by ping (layers 2 and 3). curl shows the connection to port 443 times out (layer 4). If the connection works but the server returns 500, the fault is at layer 7.

Common mistakes

• Forcing real protocols into exactly one layer; TLS is usually described as spanning 5 and 6, and the real internet follows the four-layer TCP/IP model.
• Memorizing layers without using them to troubleshoot.

Related

• TCP
• Packet
• Router
• Network switch

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