3Com 3CR17501-91 - SuperStack 3 Switch 3250, Implementation Manual

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116 A
PPENDIX
D: A
DVANCED
IP R
OUTING
C
ONCEPTS
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 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.
Step 2 - 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. See
Figure 37 .