Walk the stack from the bottom. Do not jump to “probably DNS” if the interface has no carrier.
Order
- Interface up?
- Address / mask / DHCP vs static
- Ping the gateway (if ICMP is allowed)
- DNS: name → address
- Route to the target
- Service port (local listen vs remote)
Windows
ipconfig /all · ping · tracert · nslookup · netstat -ano · Get-NetRoute
Linux
ip -br a · ping · traceroute · dig · ss -tulpn · curl -vI
Note: command, output, conclusion. Scanning someone else’s network without permission is not troubleshooting.