Classless Inter Domain Routing (CIDR)

By 1992, the exponential growth of the Internet was raising serious con 

cerns among members of the IETF about the ability of the Internet's

routing system to scale and support future growth. These problems

were related to:

  The near term exhaustion of the Class B network address space

  The rapid growth in the size of the global Internet's routing tables

  The eventual exhaustion of the 32 bit IPv4 address space 

Throughout the Internet's growth, the first two problems listed became

critical and the response to these immediate challenges was the develop 

ment of Classless Inter Domain Routing (CIDR). The third problem,

which is of a more long term nature, is currently being explored by the

IP Next Generation (IPng or IPv6) working group of the IETF. 

CIDR was officially documented in September 1993 in RFC 1517, 1518,

1519, and 1520. CIDR supports two important features that benefit the

global Internet routing system:

  CIDR eliminates the traditional concept of Class A, Class B, and Class

C network addresses. 

  CIDR supports route aggregation where a single routing table entry

can represent the address space of thousands of traditional classful

routes. This allows a single routing table entry to specify how to route

traffic to many individual network addresses. Route aggregation helps

control the amount of routing information in the Internet's backbone

routers, reduces route flapping (rapid changes in route availability),

and eases the local administrative burden of updating external rout 

ing information. 

Without the rapid deployment of CIDR in 1994 and 1995, the Internet

routing tables would have in excess of 70,000 classful routes and the

Internet would probably not be functioning today.

CIDR Promotes the Efficient Allocation of the IPv4 Address Space

CIDR eliminates the traditional concept of Class A, Class B, and Class C

network addresses and replaces them with the generalized concept of a

network prefix. Routers use the network prefix, rather than the first 3

bits of the IP address, to determine the dividing point between the net 

work number and the host number. As a result, CIDR supports the

deployment of arbitrarily sized networks rather than the standard 8 bit,

16 bit, or 24 bit network numbers associated with classful addressing. 

In the CIDR model, each piece of routing information is advertised with

a bit mask (or prefix length). The prefix length is a way of specifying

the number of leftmost contiguous bits in the network portion of each

routing table entry. For example, a network with 20 bits of network

number and 12 bits of host number would be advertised with a 20 bit

prefix length (/20). The IP address advertised with the /20 prefix could

U N D E R S TA N D I N G   I P   A D D R E S S I N G

3 2