Supernetting - 3Com 3C421600A Reference Manual

networks. So, their netmasks are actually splitting up the third octet in
their IP addresses. See Figure 3.
Figure 3 Sample CIDR Netmask
$#VDPSOH
QHWPDVN
Notice that the number of zero bits in the third octet actually dictates the
number of Class C networks in the supernet. Each zero bit makes the
supernet twice as large. So, a supernet composed of 8 Class C networks
would actually have 3 zeroes (8 = 23).
This would seem very limited since it restricts you to using groups that
nicely fit into a power of 2 (1, 2, 4, 8, 16...). However,
inconveniently-sized supernets can be accommodated because of a
simple fact: a netmask with more 1 bits will override a netmask with
fewer 1 bits.
This allows a smaller supernet to share the address space of a larger
supernet. If, for example, you had a supernet of size 6 and a supernet of
size 2, you could assign the larger supernet an 8 network address space
and assign the smaller supernet the portion of that address space that the
larger supernet was not using.
Because the smaller supernet netmask has more 1 bits, packets whose
address was part of its address space would be routed to the smaller
supernet even though the address is also part of the address space
dictated by the larger supernet netmask.
11111100
255.255.252.0
Network Portion

Supernetting

245
Dictates the number
of Class C networks
Host Portion