1.1.2 Subnet Mask

Regardless of the boundary position of net ID,host ID of Class A,B,C shown in Figure 1-1 IP Address Format, you can specify the boundary position of net ID and host ID at any boundary position using the subnet mask.

For example, if you obtain a Class B network ID and divide the network into 256 subnets, you would apply subnet mask 255.255.255.0. Or, if you want to employ CIDR and obtain two consecutive Class C network IDs (for example, 192.0.0.x and 192.0.1.x) to create a single subnetwork, you would apply subnet mask 255.255.254.0.

Specify a subnet mask in the configuration for each interface in left-adjusted format (specify consecutive 1s from the most significant digit in binary notation).

For example, you can specify 255.255.192.0 as a subnet mask, but you cannot specify 255.255.96.0 as a subnet mask.