The Class B network 128.10.0.0 can be subnetted using the first 8 bits of the host-id, to span 254 different physical networks.
The subnet mask for this case is 255.255.255.0
The subnetworks are: 128.10.1.0, 128.10.2.0, ..., 128.10.254.0 .
Each of the subnetworks can have up to 254 different hosts:
128.10.XXX.1, 128.10.XXX.2, ..., 128.10.XXX.254 .
If there is a need for less physical nets and more hosts in each one, less host-id bits can be used for subnetting.
For example:
With the subnet mask 255.255.254.0,
126 different subnets are available with up to 510 hosts in each one.
Many Class A and B networks do not contain as many hosts as they could. This situation causes a lot of address space waste.
See Next page for corresponding diagram.