Do you mean, say, one device that's on Ethernet and one that's on WiFi? Most people have their Ethernet and WiFi setups bridged so they count as the same link.

Do you mean, say, one device that's on Ethernet and one that's on WiFi? Most people have their Ethernet and WiFi setups bridged so they count as the same link. If you actually mean two separate networks, where traffic has to be routed via the router... obviously there's a lot of ways of setting up firewall rules, and it depends on exactly what you want to do, but the simplest approach is to say "deny new connections coming in via the WAN interface". This will allow new connections going out the WAN interface, and also allow all connections between the two local networks in both directions, with no need to specify any IP ranges in the firewall at all.