| By using a bitmask instead of an
address class to determine the network portion of an address, CIDR
allows routers to aggregate, or summarize, routing
information. This shrinks the size of the router's routing tables.
In other words, just one address and mask combination can represent
the routes to multiple networks.
Without CIDR and route aggregation, a
router must maintain individual entries for the Class B networks
shown in Figure .
The shaded columns in Figure
identify the 16 bits that, based on the rules of class, represent
the network number. Classful routers are forced to handle Class B
networks using these 16 bits. Because the first 16 bits of each of
these eight network numbers are unique, a classful router sees eight
unique networks and must create a routing table entry for each.
However, these eight networks do have common bits, as shown by the shaded
portion of Figure .
Figure
shows that the example eight-network addresses have the first 13
bits in common. A CIDR-compliant router can summarize routes to
these eight networks by using a 13-bit prefix, which these eight
networks, and only these networks, share these bits:
10101100 00011
To represent this prefix in decimal
terms, the rest of the address is padded with zeros and then paired
with a 13-bit subnet mask:
10101100 00011000 00000000 00000000
= 172.24.0.0
11111111 11111000 00000000 00000000 = 255.248.0.0
Thus, a single address and mask
define a classless prefix that summarizes routes to the eight
networks, 172.24.0.0/13.
By using a prefix address to
summarize routes, you can keep routing table entries manageable,
which results in the following:
- More efficient routing
- A reduced number of CPU cycles
when recalculating a routing table or when sorting through the
routing table entries to find a match
- Reduced router memory requirements
Supernetting is the practice of using
a bitmask to group multiple classful networks as a single network
address. Supernetting and route aggregation are different
names for the same process, although the term supernetting is
most often applied when the aggregated networks are under common
administrative control. Supernetting and route aggregation are
essentially the inverse of subnetting.
Recall that the Class A and Class B
address space is virtually exhausted, leaving large organizations
little choice but to request multiple Class C network addresses from
their providers. If a company can acquire a block of contiguous
(that is, sequential) Class C network addresses, supernetting can be
used so that the addresses appear as a single large network, or supernet.
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