You don't debug "the internet." You debug which layer failed — name, port, route, or cable.

L7 Application HTTP · DNS · TLS · SSH

L4 Transport TCP · UDP · ports

L3 Network IP · ICMP · routing

L2 Data link MAC · ARP · Ethernet

L1 Physical cable · Wi‑Fi · NIC

• TCP/IP — model you actually use
• 5 — layers worth memorizing
• L7 — where most vulns live

Symptom

• Site loads forever → often DNS or TCP
• Connection refused → port closed / no listener
• Timeout → filter, blackhole, or no route

First question

• Did the name resolve?
• Is the port open from here?
• Do I have a route to that IP?