The growing demand
for IP addresses has put a strain on the classful model, especially
class B address space, which was getting depleted at a fast pace.
One of the measures to handle the IP depletion is the introduction
of CIDR (Classless Inter-domain Routing).
CIDR is a move away from traditional IP classes A/B/C. In CIDR, an
IP network is represented by a prefix, which is an IP address and
some indication of the left-most contiguous significant bits within
this address. CIDR uses extended network prefixes between the
left-most 13 and 27 bits, comparing with the traditional class A/B/C
structure which is limited to 8/16/24 bits respectively.
For example, 198.32.0.0 used to be an illegal class C network, is
now a valid prefix with a notation 198.32.0.0/16. The /16 is an
indication of using 16 bits of mask counting from the far left. A
network is called a supernet when the prefix boundary contains fewer
bits than the network’s natural mask.
| CIDR Block Prefix | Equivalent Class | Number of Hosts | |
| /27 | 1/8 of Class C | 32 | |
| /26 | 1/4 of Class C | 64 | |
| /24 | 1 Class C | 256 | |
| /16 | 1 Class B | 65,536 | |
| /13 | 8 Class B | 524,288 |
In
recent years, the IP routing tables held in the Internet routers
have grown in a way that caused routers to start being saturated as
far as processing power and memory allocation. CIDR is the technique
supported by BGP4 (Border Gateway Protocol version 4) and based on
route aggregation. CIDR allows routers to group routes together in
order to cut down on the quantity of routing information carried by
the core routers. With CIDR, several IP networks appear to networks
outside the group as a single, larger entity.
Routing domains that are CIDR-capable are called classless domains,
in contract to the traditional classful routing domains. CIDR has
depicted a more hierarchical Internet architecture, where each
domain takes its IP addressed from a higher hierarchical level. This
gives tremendous savings in route propagation especially when
summarisation is done close to the so-called leaf networks. Leaf
networks are endpoints on the global network; they do not provide
Internet connection to other networks. An ISP that supports numerous
leaf networks subdivides its subnets into many smaller blocks of
addresses to serve their customers. Aggregation permits the ISP to
advertise the addresses in a single notation rather than many, thus
resulting in more efficient routing strategies and propagation.
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