If you use the 192.168.0.x range as seen in these examples, you can assign any numbers to your machines between 192.168.0.1 & 192.168.0.254.
The first and last addresses, 192.168.0.0 & 192.168.0.255, are already used by the network and cannot be assigned. But all other numbers in the range are available for use.
Additionally, all machines on the same network need to use the same subnet mask to be able to communicate directly with each other.
The netmask is labeled "optional" in this panel because if it is not specified, 255.255.255.0 will be used as the default.