For the 2 conditions you laid out.
a . network of 192.168.0.x
b. A subnet of 192.168.18.1 as the default gateway.
you can try a subnet mask of 255.255.128.0
that will give you a network with a range of 192.168.0.1 thru 192.168.127.245
both of your conditions would be within that range and this would be 1 network.