IP and Gateway Problems: Addresses, Masks, Gateways and Routes

A real-life situation

A new printer was given a static IP address by hand. Everyone on the same floor can print to it, but people in the other building can't. The printer works, the cables work and the switches work. The fault is one wrong number in its settings: the default gateway.

What goes wrong at Layer 3

Fault	Typical symptom	How to spot it
Wrong IP (in the wrong subnet)	Can't even reach the gateway	Compare the host address with the gateway: same subnet?
Wrong mask	Some local devices work, others don't	Compare the mask with the mask on the gateway's interface
Wrong or no gateway	Local traffic works; other networks don't	`ipconfig` / `ip route`
Duplicate IP	Connection keeps dropping and coming back	Windows warning, `%IP-4-DUPADDR` log message on Cisco routers
Missing route	One remote network fails, others work	`show ip route`, traceroute

Why the gateway matters

A host compares the destination with its own address and subnet mask (the value that shows which part of the address is the network). If the destination is in the same subnet, the host sends the packet straight to it. If not, it sends the packet to its default gateway. So a wrong gateway only breaks traffic to other networks. That is exactly what happened with the printer.

1. Same subnet: the PC sends straight to the printer. No gateway is needed, so printing works.
2. From building B: the print job reaches the printer through R1.
3. The reply goes nowhere: the printer tries to send it to gateway 192.168.10.254, which does not exist, so the reply is never delivered. Fix: set the gateway to 192.168.10.1.

How to verify: the host side

Example output · typical of Windows, written for this lesson, not captured from a real computer

C:\> ipconfig
Ethernet adapter Ethernet:

   IPv4 Address. . . . . . . . . . . : 192.168.10.40
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.10.1

What to look for: check three things. Is the IPv4 Address in the same subnet as the gateway? Does the Subnet Mask match the mask on the gateway's interface? Is the Default Gateway the router's real address? Here all three are correct, so ping the gateway next. On Linux, use ip addr and ip route.

Missing routes, and the return path

Each router on the way needs a route to the destination, and each router on the way back needs a route to the source. A ping only works if both directions work. Forgetting the return route is one of the most common mistakes when adding static routes.

1. Going there works: R1 has a static route to 192.168.30.0/24.
2. Coming back fails: R2 has no route to 192.168.10.0/24, so it drops the reply. The PC only sees a timeout.

Based on Cisco documentation, not run on a lab device:

Example output · based on Cisco documentation; exact format varies by platform and software version

R2#show ip route 192.168.10.0
% Network not in table

What to look for: % Network not in table means R2 has no route to the PC's network. Without a default route either, R2 drops every reply addressed to 192.168.10.0/24.

Example output · based on Cisco documentation; exact format varies by platform and software version

R2(config)#ip route 192.168.10.0 255.255.255.0 10.0.12.1

The fix: a static route back to the PC's network, using R1's address on the shared link (10.0.12.1) as the next hop. Then test again from R2 with ping 192.168.10.40 source 192.168.30.1. Using R2's LAN address as the source tests the same return path that the server's replies use.

Check yourself

Predict · scenario 1

A PC can reach every device in its own subnet but nothing outside it. What is the most likely fault?

Predict · scenario 2

A PC has 192.168.10.40/24 and gateway 192.168.1.1. What is wrong?

Predict · scenario 3

R1 has a route to the server's network. Traceroute from the PC shows R1 as hop 1, then only stars. What should you check on R2?

IP and gateway problems