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Classless Addressing
Mathematically, the IPv4 address space
still held a substantial number of available addresses. Unfortunately,
many of these potential addresses were squandered because they were locked
into assigned blocks, or classes, of assigned addresses. Eliminating
classes wouldn't necessarily recover the addresses locked into those
address spaces that were already assigned, but it would enable the
remaining addresses to be used much more efficiently. Ostensibly, this
stopgap effort would buy the time needed for IPv6 to be developed and
deployed.
Enhanced Route Aggregation
CIDR enables Internet routers (or any CIDR-compliant
router) to more efficiently aggregate routing information. In other words,
a single entry in a routing table can represent the address spaces of many
networks. This can greatly reduce the size of the routing tables that are
needed in any given internetwork and directly translates into an increased
scalability.
CIDR was implemented in the Internet during
1994 through 1995 and was immediately effective in containing the
expansion of the Internet routers' routing tables. It is doubtful that the
Internet would have continued to grow had CIDR not been implemented.
Supernetting
Another benefit of CIDR is the capability
to supernet. Supernetting is nothing more than using contiguous
blocks of Class C address spaces to simulate a single, albeit larger
address space. If you were to obtain enough contiguous Class C addresses,
you could redefine the allocation of bits between network and host
identification fields and simulate a Class B address.
Supernetting is designed to alleviate the
pressure on the rapidly depleting Class B address space by offering a more
flexible alternative. The previous class-based address architecture
suffered from a tremendous disparity between its Class B and C networks.
Networks that required more than the 254 hosts offered by a Class C had
the following two choices, neither of which was highly desirable:
- Using multiple Class C addresses
(which would have necessitated routing between the network domains)
- Stepping up to a Class B address with
its 65,534 usable host addresses
The simpler solution, frequently, was to
use the Class B even though it wasted tens of thousands of IP addresses.
Consider a company (Catco, Inc.) that has been granted the following registered Class C addresses: 220.220.1.0 through 220.220.255.0. Assume that Catco has assigned (used) 220.220.1.0 through 220.220.100.0. Catco does not need to advertise the entire list of 100 individual Class C addresses to the Internet. Instead, Catco can supernet the addresses to a shorter prefix: 220.220.0.0/16. This results in a 99 percent reduction in the number of routes that need to be advertised to the Internet. Full connectivity from the Internet to the 100 assigned Class C addresses is still possible because Catco has control of every possible network with the 16-bit supernet prefix of 220.220.0.0 (220.220.1.0, 220.220.2.0, . . . , 220.220.254.0 and 220.220.255.0).
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