PeteDotCom -> Networking -> TCP/IP -> Subnet Mask ->   A subnet mask is a 32-bit number, written in dotted decimal form, in much the same way an IP address is displayed. For example  255.255.255.192. Only hosts that are on the same subnet can communicate with others without a router. Computers on different subnets can not see each other’s local broadcasts, and they will need to communicate to each other through a router, even though they may be on the same network. An example of how a subnet marks works, is say for example you have a class c network address, and you would prefer to configure six networks of 30 hosts each, you would use the subnet mask 255.255.255.224. Subnet 0, with the binary representation 000, is reserved to refer to “this subnet,” and subnet 7, with the binary representation 111, is reserved for broadcasts to all subnets on this network. Thus subnets one through six are available. Host number 0 and host number 31 are reserved, so there are 30 potential host IDs, numbered 1 to 30, that could be assigned. With this subnet mask, there are 180 total IP addresses possible on this Class C network. With no subnetting, a Class C network has 254 total addresses. Subnets clearly can result in wasted IP addresses, although the alternative of not employing subnets at all will likely result in even worse waste—for example, multiple Class C networks being used to segment a small number of users.