Can reach the local network but not other subnets

Likely causes

1. No default route (default via <router> missing from ip route).
2. A prefix too short, such as /16 on a /24 network: the machine thinks far addresses are on its own wire and asks for them with ARP.
3. A default route via an address nobody answers for.

Diagnose

Run these in order. The output is what each printed on a lab's real Linux machines.

laptop:~# ip route

Only the local network, 10.0.1.0/24. No default line: nothing says where other networks are.

10.0.1.0/24 dev eth0 proto kernel scope link src 10.0.1.10 

alice:~# ping -c2 10.0.2.10

With a /16 address, Destination Host Unreachable comes from the machine's own address: it looked for 10.0.2.10 on its own wire.

PING 10.0.2.10 (10.0.2.10) 56(84) bytes of data.
From 10.0.1.10 icmp_seq=1 Destination Host Unreachable
From 10.0.1.10 icmp_seq=2 Destination Host Unreachable

--- 10.0.2.10 ping statistics ---
2 packets transmitted, 0 received, +2 errors, 100% packet loss, time 1028ms
pipe 2

alice:~# ip route get 10.0.2.10

On a healthy machine, ip route get names the router: via 10.0.1.2.

10.0.2.10 via 10.0.1.2 dev eth0 src 10.0.1.10 uid 0 
    cache 

Fix

laptop:~# ip route add default via 10.0.1.2

Add the default route through the router.

laptop:~# ip route

default via 10.0.1.2 is now in the table.

default via 10.0.1.2 dev eth0 
10.0.1.0/24 dev eth0 proto kernel scope link src 10.0.1.10 

laptop:~# ping -c1 -W1 10.0.2.10

The far network answers.

PING 10.0.2.10 (10.0.2.10) 56(84) bytes of data.
64 bytes from 10.0.2.10: icmp_seq=1 ttl=63 time=0.154 ms

--- 10.0.2.10 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.154/0.154/0.154/0.000 ms

alice:~# ip addr del 10.0.1.10/16 dev eth0

For a wrong prefix: delete the address with the wrong prefix (the right one stays).

alice:~# ping -c1 -W2 10.0.2.10

The server answers again, through the router.

PING 10.0.2.10 (10.0.2.10) 56(84) bytes of data.
64 bytes from 10.0.2.10: icmp_seq=1 ttl=63 time=0.091 ms

--- 10.0.2.10 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.091/0.091/0.091/0.000 ms

Break it and fix it yourself

Each lab is a small network of real Linux machines in your browser, where this fault happens.

• A bare machine: networking by hand Free: its challenge "A gateway nobody answers for" set the fault for you to find and fix.
• Addresses and subnets: on my network, or through the router? Free: its challenge "Bob can't reach the file server" set the fault for you to find and fix.

Open A bare machine: networking by hand