My CISCO Journal

PART I:

CIDR (RFC: 4632): Classless inter domain routing. In simple words, CIDR is like supernetting route summarization and VLSM at the ISP end that’s why it is called Classless inter domain routing. Confusing? Okay let us discuss it via an example; it will help us picture the whole concept. As we learned in route summarization, instead of advertising each address, a router makes a block of addresses and advertise it, it minimizes load on routers and enhances network efficiency.

Instead of assigning addresses according to the Classful subnet boundaries, the ISPs begun to assign IP addresses in the form of blocks. Then it was the duty of the ISPs to assign smaller blocks to its customers from the Big block. In CIDR the routers were given the ability to process the IP addresses according to the classless subnet prefix instead of the starting 0 and 1s in each IP address. Or in the other words, the routers were programmed to understand the prefix through which it can decide to which domain (major block of IPs) these addresses are assigned.  

If the a whole Class A, B or C address is assigned to an organization, there is a chance of the wastage of IP addresses, so IP address conservation was one of the main reasons behind CIDR development by IANA. IANA suggested assignment of IPs address ranges other than the conventional class paradigm. These policies not only helped in preserving the wastage of IP addresses but also reduced the load of the global routing tables.  So the two goals behind CIDR creation by IANA were:

Let understand the first point via an example. Suppose we have a block of addresses are below:

192.168.20.0/24

192.168.21.0/24

192.168.22.0/24

192.168.23.0/24

We can summarize this address as:

Our block size is 4, so our best subnet choice at 3^rd octet is 256-4 = 252, so we can summarize the above network IDs with following subnet IP:

192.168.20.0

255.255.252.0 OR

192.168.20.0/22

Have you noticed one interesting thing, we have moved backward from standard Class C boundary. We have an IP address 192.168.20.0 with the prefix 22 which represents a block of 4 IP addresses of 24 subnets. Now the router will only advertise with /22 addresses and the same process are repeated from our side to our ISP. Our ISP does the same route aggregation and advertises a single block to higher networks and the process goes on.  Without this process we would have millions of routing tables for the global IP traffic! And how the IP addresses are conserved? Well they are conserved when a specific block of IP addresses is assigned to customers. 

An ISP will never assign a Classful address to any corporation, as a lot of addresses will go waste and the ISP can’t take back the non-used addresses. For the solution of this problem, IP address Ownership an IP address Lending scheme was introduced but they are still in review amidst much heated debate on it.