Firewall zones

I want to configure the SuSE firewall so that communication within my LAN is uninhibited but communication outside the LAN is fully protected. Looking at the firewall configuration in Yast, I see that the external zone is protected but the internal zone is not. However, I don't see how to specify that the internal zone consists of hosts with addresses 192.168.0.x. This would seem to be a pretty common requirement. It appears that the firewall configurator can specify that an interface is external or internal, but I have only one interface (network card). It connects to the LAN and to the router; the router in turn talks to the world. It's a very common setup. Paul