I am going to assume you are using a IP address VERY similar to 192.168.1.x most NAT devices make 192.168.x.x subnets for your local hosts.
lets pretend your DSL router is 192,168,1,1 and that it gives out addresses from 192.168,1,2 to 192.168.1.254 for your local subnet.
in this scenario and address in side 192.168.1.x is directly addressable and does not need the router so the subnet has 0s in the last octet x.x.x.0 Any address out side 192.168.1.x does need the router to relay them to another subnet so all the upper 3 octets are 1s 255.255.255.x since that describes all 4, 255.255.255.0 we are done.
Life gets trickier if you want subnets with boundries not at the dots. ie bigger than 256 but less than 16 million. I tried to remain child-like, all I acheived was childish.