Installing and Administering LAN/9000 Software

Chapter 6 167
Network Addressing
CIDR - Classless Inter-Domain Routing
CIDR - Classless Inter-Domain Routing
As the Internet has evolved and grown in recent years, it has become
clear that it is facing several serious problems. These include:
• Exhaustion of the Class B network address space. One fundamental
cause of the problem is the lack of a network class of a size that is
appropriate for a mid-sizedorganization. Class-C, with a maximum of
254 host addresses, is too small, while Class-B, which allows up to
65534 addresses, is too large to be densely populated. The result is
inefficient utilization of Class-B network numbers.
• Routing Information overload. The size and rate of growth of the
routing tables in Internet routers is beyond the ability of current
software (and people) to effectively manage.
Classless Inter-Domain Routing (CIDR) attempts to deal with these
problems by defining a mechanism to slow the growth of routing tables
and reduce the need to allocate new IP network numbers.
The basic idea of the CIDR plan is to allocate one or more blocks of
Class-C network numbers to each network service provider.
Organizations using the network service provider for Internet
connectivity are allocated bitmask-oriented subnets of the provider’s
address space as required.
Figure 6-16 Internet Address 192.6.13.2 ANDed with Supernet Mask