I just had a problem where the computers in my house that were all using a wired connection could not talk to each other. Computers on wired connections could not ping, and ping kept giving an error saying “destination host unreachable”. I found the solution in a very hard to find forum post on the dd-wrt forums.
This specific instance of the problem affects users of the dd-wrt firmware. In my case, I am using build 24461 of the firmware. The solution is to run the following commands on the router (either with ssh or telnet):
swconfig dev eth0 set enable_vlan 1
swconfig dev eth0 set apply
I’ve since put this in the startup commands that the router will execute, so these settings will stay even after a router reboot.
For some routers, it might be eth1 instead of eth0. It depends on which network interface is assigned to the WAN. This can be check in the router page under Setup > Networking.