A netmask is a 32-bit mask used to divide an IP address into subnets and specify the networks available hosts. In a netmask, two bits are always automatically assigned. For example, in 255.255.225.0, “0″ is the assigned network address; and in 255.255.255.255, “255″ is the assigned broadcast address. The 0 and 255 are always assigned and cannot be used.
Because of VM setup I couldn’t emulate this on back track but I used something similar like this in ubuntu.