The OSI model can be your friend if you don’t take it too seriously, and just implement the useful parts. The useful parts to a network troubleshooter are Physical (1), Data Link (2), Network (3), and Application (7). The Physical layer includes everything from the physical world; the Data Link layer usually involves the signaling from your Ethernet interface (but could also be the communications from your wide-area link or other network type). The Network layer means TCP/IP (or other protocol) connectivity; and for troubleshooting purposes, you can consider the Application layer as everything else. When troubleshooting a complex problem, you’ll want to start with layer 7 and move backward to layer 1.