The default gateway is the router address a host uses to reach another network. Without it, a PC can only communicate inside its own local network.

A troubleshooting routine

• Check the PC IP settings (address, mask, gateway).
• Test the gateway with a local ping.
• Check the state of the interfaces on the router.
• Work your way outwards towards the remote destination.

C:> ping 192.168.1.1
R1# show ip interface brief
R1# show ip route

A failed ping to the gateway usually points to a wrong mask, a wrong gateway or an interface that is down. The show ip route command reveals the networks the router actually knows about. Work layer by layer, from the nearest point outwards, to isolate the fault.

Key point: no default gateway means no communication between networks; troubleshoot from local to remote. See Lab 9 – Troubleshooting basic connectivity.