From a command prompt ( Start, Run , cmd ) key ipconfig Look at IP for default gateway. It will be summat like 192.168.1.1
In Z/Alarm select Firewall + then Zones tab. At bottom right select Add then IP Range. Enter a trusted zone range of 192.168.1.1 - 192.168.1.4 ( this would consider 3 hosts in your network as "Trusted" ).