so where are we using the CIDR? how can we handl the CIDR block? any real world scenario?

Hi,

CIDR is mainly used by ISPs to summarize routes and thereby limiting the size of routing tables on the Internet routers.

However, there is no limitation to use CIDR on private ip addresses within your company, if you use hierarchical addressing and you want to use summarized routes within your private domain.

Cheers:

Istvan

Dan

There have been several good posts and I would like to approach this from a slightly different perspective. As others have pointed out, subnetting is a way to break up a network into smaller (and more useable) parts. I suggest that CIDR is one way to think about how we do the subnetting process. For me it helps to think about the term CIDR and to focus on the first and last words - it is about classless routing.

In the early days of subnetting the approach was very classful. All subnets of a network were required to be the same size and we were constrained (at least in some respects) by the "class" of address that we were assigned. CIDR (and its associated concept of VLSM) gave us a different way of thinking about how to create and how to use subnets. In this environment it is more important to think about what size address space do you need (is it /27, or 22, or 20) and less about whether it is an A, or a B, or a C.

HTH

Rick

Rick,

If I want to explain difference between Subnetting and CIDR then how I can explain to them, I am seriously confuse about it.

can i say taht subnetting you can have in perticular Class network but CIDR is applied to any network ID as per your requiremnt no matter which class IP you are using. I can use 192.0.0.0/10 CIDR eventhough it is fall under the Class C range? does it make sense?

Dan

I believe that it is confusing in part because you want there to be more difference than there really is. As a way of illustrating this let me ask what may seem an odd question: what is the difference between automobile and Ford (or Toyota or VW)?

Well they are not really different because Ford (or whatever) is one way to do automobile and there are other waus to do automobile. So subnetting is a way to organize groups of addresses so that they become more useful. CIDR is a particular approach to how to organize addresses so that they become more useful.

Or perhaps a somewhat different way to explain it: CIDR is closely related to VLSM. Perhaps we can say that CIDR tries to do on a larger (network) scale what VLSM tries to do on a smaller (subnet) scale.

HTH

Rick

Rick,

some time small things are big enough to jumble us. but again what ever I have posted in my last post about one example of 192.0.0.0/12 is correct or not? I got your point is VLSM and Classless both are usefull in terms of IP address management as well as Summurisation so I think using summurisation we can describe Classless more effectively as in large network it will reduce the routing table size at great extent what is your thoughts on it?

Dan

In your previous post you asked:

I can use 192.0.0.0/10 CIDR

That is certainly an example of what is frequently called supernetting and supernetting is an important component of CIDR.

HTH

Rick

Rick,

how about this one?

"I got your point is VLSM and Classless both are usefull in terms of IP address management as well as Summurisation so I think using summurisation we can describe Classless more effectively as in large network it will reduce the routing table size at great extent what is your thoughts on it?"

thanks Rick for informative conversation thanks again and your replies as well as replies from other experts help me a lot in day to day work as well as to make my fundamental solid.

Dan

I am glad that we have been able to help you. Thanks for using the rating system to rate responses that you find helpful.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick