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OSI model in practice

Use the seven layers as a troubleshooting checklist, not a memorization chant.

The OSI model is a shared vocabulary. Real networks run TCP/IP, but when a circuit is down you still walk layers: physical, link, network, transport, then the application.

• L1 — cables, SFPs, interface up/up, error counters.
• L2 — MAC tables, VLANs, trunks, spanning tree.
• L3 — IP addresses, gateways, routes.
• L4 — TCP/UDP ports, state, ACLs that match them.
• L5–L7 — the payload: HTTP, DNS, SSH.

If ping fails, do not start in the web server logs. Confirm the interface is up, the VLAN is correct, and the gateway answers. Then move up.