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-sized organization. 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 255.255.254.0

To implement this feature, you must apply the supernet netmask to all interfaces connected to the supernet using the ifconfig(1M) command. This feature will allow all hosts on the supernet to communicate with all other hosts on the supernet without standard routing.

In the example below, the hosts in two Class C networks, 192.6.12 and 192.6.13, are included in a larger supernet using the netmask option.




	NOTE: Since the gateway is itself a host on the supernet, it must support CIDR.

Figure 6-17 Network Map for Supernetting

CIDR - Classless Inter-Domain Routing

Technical documentation

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