The subnet mask defines how many bits after the network ID will be used for the subnet ID. The subnet ID can vary in length, depending on the value you select for the subnet mask. As the subnet ID grows larger, fewer bits are left for the host ID. In other words, if your network has many subnets, you will be limited to fewer hosts on each subnet. If you have only a few subnets and require only a few bits for the subnet ID, you can place more hosts on a subnet.
By the Way
Note that the address class also defines how many bits will be available for the subnet ID. The mask
specifies 19 bits for the network ID and subnet ID together. If this mask is used with a Class B address (which has a 16-bit ...