User manual

B-8 Addressing Scheme
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This example creates supernets composed of fewer than 254
Class C networks. So, their netmasks are actually splitting up
the third octet in their IP addresses. See Figure 4 below.
Figure 4. Sample CIDR Netmask
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 = 2
3
).
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’s 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’s netmask.