 FACT: Subnet Masks: When Jack checks Palmers computer, the screen  shows the subnet mask address identical to his IP address. Subnet  mask addresses always start with the digit 255 at the beginning of  the four digits (the exception being the subnet mask address  0.0.0.0.). Also subnet masks can only have the following numbers as  well as 255 and 0: 192, 224, 240, 248, 252 and 254. Example:  255.255.255.192 would be considered as a real subnet mask.