Classless Interdomain Routing
As we have just seen VLSM breaks down a Classful IP address range to create more IP networks with fewer IP addresses. This is ideal for small organizations or those who want to divide up their network into segments to reduce broadcasts. However, what happens if a larger organization needs more IP addresses for a single network segment? This can be done by combining IP address ranges together using a technique called Classless Interdomain Routing (CIDR).
Using CIDR, multiple subnets can be combined to create a single logical network of IP addresses. There are certain limitations, due to routing protocols and hardware devices.
If you want to find out whether or not two IP address ranges can be combined using CIDR, you first need to ascertain whether they share the higher order bits. Consider the following examples:
Example 1 Bits
Example 2 Bits
Example 1 can use CIDR, because the first 23 bits are the same, and can therefore be combined into a single network by the use of a classless network with a subnet mask of 255.255.254.0. This would produce a new subnet with 510 host addresses.
Example 2 cannot use CIDR because only the first 14 bits are the same.
Next: Configuring TCP/IP