USRobotics NETServer/8 - Manual (Page 236)

B-8 Addressing Schemes
Step two - Select a range of addresses for each supernet
The range of addresses in a supernet must fit exactly into a
space that can be described by its netmask. This means that the
zero bits in the netmask must also appear in the first address of
the supernet block. For this to be true, the third octet in the
address must be an even multiple of the same power of 2 used
to form the netmask. For example, if you had created a block of
8 networks, the third octet in the first address will be an even
multiple of 8.