CIDR Classless Inter-Domain Routing Tutorial

In this Cisco CCNA training tutorial, you’re going to learn about CIDR, which is Classless Inter-Domain Routing. Scroll down for the video and text tutorial.

CIDR Classless Inter-Domain Routing Video Tutorial

YouTube video


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A problem with the original implementation of the classful addresses was that when the Internet authorities give out addresses, they always give out complete:

  • Class A with a /8 subnet mask
  • Class B with a /16 subnet mask
  • Class C with a /24 subnet mask


CIDR Classless Inter-Domain Routing


This gave a problem that if a company had more than 254 hosts, let’s say 500 hosts, is too big for a Class C. So, they would be given a Class B and they would get allocated addresses for 65,534 hosts.


That's way too much and this led to huge amounts of the global address space being wasted. As a solution, Classless Inter-Domain Routing or CIDR was introduced in 1993.


CIDR removed the fixed /8, /16, and /24 requirements for the different address classes and allowed them to be split in smaller networks called subnetting.


CIDR Classless Inter-Domain Routing 1


For example, the Internet authorities could allocate the address, /20. The first octet, 175, is a Class B address which would normally be /16. Rather than allocating the entire /16, the Internet authorities could now assign /20 which means the other networks in the 175 range would be available to give to other companies.


Rather than giving a huge range, we split the classes into smaller networks that could be given to different organizations. Companies can now be allocated in an address range that matches what they need, therefore, there would be fewer addresses getting wasted.


CIDR and Route Summarization


Route summarization is another benefit of CIDR. In the example below, we've got ISP A and they have allocated the address blocks that you see on the left. One company got /24, another one got /24, one got /24, all the way up to /24. ISP A has given out 255 address blocks.


We've also got ISP B and they've given out /24, /24, and so on, all the way up to /24. ISP A and ISP B get connected.


CIDR and Route Summarization


If we didn't have CIDR and route summarization, ISP A would advertise all of its 256 address blocks to ISP B and vice versa. But when we've got CIDR and route summarization, the two ISPs can advertise just an aggregate block.


Rather than advertising all 256 /24s, ISP A advertises /16 which is a superset of all those 256 smaller networks. ISP B then learns one route to all the networks behind ISP A, rather than learning 256 routes. ISP B will similarly advertise one route of /16 to ISP A.


Route Summarization Benefits


The benefits we get from route summarization is that ISP A doesn't know about all 256 networks behind ISP B. It only gets a single summary route that covers all of them.


One route compared to 256 routes, is a lot less information, more efficient, and it takes up less memory in the router. If an individual link goes down in ISP B, it doesn't have any impact on ISP A because that one summary route doesn't change.


Route Summarization Benefits


It's going to be different in ISP B though. Whenever one of their routes goes down, the other routers are going to have to recalculate that.


The benefit we get from this is we're compartmentalizing the different parts of our network. If we've got an issue in ISP A of the network, it's not going to impact ISP B. It makes things a lot more stable and reliable. It also makes things more logical, which is better for us humans because it makes it easier to troubleshoot related problems.


Additional Resources

Subnetting practice questions.

Understanding CIDR Subnet Mask Notation:

Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan:

IP Addressing and Subnetting for New Users:


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Libby Teofilo

Text by Libby Teofilo, Technical Writer at

With a mission to spread network awareness through writing, Libby consistently immerses herself into the unrelenting process of knowledge acquisition and dissemination. If not engrossed in technology, you might see her with a book in one hand and a coffee in the other.