OSI Model
A seven-layer conceptual framework that standardizes how network functions communicate, from physical signaling to applications.
The OSI model divides network communication into seven layers: Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), and Application (7). Travelling down the stack, each layer wraps the unit above it in its own header — encapsulation — which the receiving side strips off on the way up. On the N10-009 exam, the key confusion is OSI versus the TCP/IP model, which collapses these seven layers into four. Troubleshooting questions ask which layer a device works at: switches at Layer 2, routers at Layer 3, TCP/UDP at Layer 4. TLS encryption maps to Layer 6, not Layer 7.
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