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2.2 | ![]() |
IP Addressing Crisis and Solutions | |
2.2.3 | ![]() |
Route aggregation and supernetting |
CIDR allows routers to aggregate, or summarize, routing information.
It does this by using a bit mask instead of an address class to
determine the network portion of an address. This shrinks the size of
the routing tables used by the router. 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 many individual entries for
the Class B networks.
The shaded columns in Figure
Figure
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:
Therefore, 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, routing table entries can be kept more manageable. The following benefits are a result of the summarized routes:
Supernetting is the practice of using a bit mask to group multiple classful networks as a single network address. Supernetting and route aggregation are different names for the same process. However, the term supernetting is most often applied when the aggregated networks are under common administrative control. Supernetting takes bits from the network portion of the network mask, whereas subnetting takes bits from the host portion of the subnet mask. Supernetting and route aggregation are essentially the inverse of subnetting. Recall that the Class A and Class B address space is almost exhausted, leaving large organizations little choice but to request multiple Class C network addresses from providers. If a company can acquire a block of contiguous, Class C network addresses, supernetting can be used so that the addresses appear as a single large network, or supernet.
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