A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
OSI is not a law, it's a checklist. When 'app is slow' you ask: cable? (L1), VLAN? (L2), route? (L3), port blocked? (L4), login/TLS? (L5-7). Check bottom-up and you stop guessing.
| # | Name | Example | If broken you see |
|---|---|---|---|
| 7 | Application | HTTP, DNS, SSH | Wrong URL, login fails |
| 6 | Presentation | TLS, UTF-8 | Cert error, gibberish text |
| 5 | Session | Login session | Timeout, need re-login |
| 4 | Transport | TCP, UDP + ports | Connection refused, port blocked |
| 3 | Network | IP, ICMP, routers | No route, ping fails across nets |
| 2 | Data Link | Ethernet, WiFi, VLAN | Wrong VLAN, no link |
| 1 | Physical | Cable, fiber, radio | No lights, flapping link |
Internet really runs TCP/IP with 4 layers: Link (OSI 1+2), Internet (OSI 3), Transport (OSI 4), Application (OSI 5+6+7). Exam asks this mapping directly — Link = 1+2, App = 5-7.
Down the stack each layer wraps the one above: App data -> TCP adds ports (segment) -> IP adds IPs (packet) -> Ethernet adds MACs + FCS (frame) -> bits on wire. Up the stack on receipt, unwrap in reverse.
Ping proves L1-L3. nc to port 22 proves L4 too. If ping works but nc fails, fault is L4 (ACL/firewall), not cable. That's layered troubleshooting.
You now debug bottom-up. Next: how office topologies and building cabling really look.
Finish the lesson once you have worked through the material. This awards ★ 35 XP.