CIDR and Subnetting

[dangal.43]

what is difference between CIDR and subnetting? to me it seems to be the same but the way of writing is different, please correct me if I am wrong.

[Edison Ortiz]

Here is a good article on this matter

http://en.wikipedia.org/wiki/Supernet

HTH,

__

Edison.

[massimiliano.serafino]

Hi

"Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network. If you do not subnet, you are only able to use one network from your Class A, B, or C network, which is unrealistic. "

"Classless Interdomain Routing (CIDR) was introduced to improve both address space utilization and routing scalability in the Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.

CIDR moves way from the traditional IP classes (Class A, Class B, Class C, and so on). In CIDR , an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. Length means the number of left-most contiguous mask bits that are set to one. So network 172.16.0.0 255.255.0.0 can be represented as 172.16.0.0/16. CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network 172.16.0.0/16, then the ISP can offer 172.16.1.0/24, 172.16.2.0/24, and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise 172.16.0.0/16."

Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml#cidr

I hope this helps.

Best regards.

Massimiliano.

[Istvan_Rabai]

Hi Dan,

Subnetting is moving the subnet mask size to the right.

CIDR is moving the subnet mask size to the left.

Subnetting:

Let's suppose you have a classful network:

192.168.1.0/24

If you want to subnet it, you can use a 25 bit or 26 bit etc. subnet mask. The mask size is moving to the right.

CIDR:

Let's suppose you have 4 classful networks:

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

You can create a summary address that will include the 4 networks in 1:

192.168.0.0/22

The subnet mask size moved to the left.

Cheers:

Istvan

[dangal.43]

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

[Istvan_Rabai]

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

[Richard Burts]

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

[dangal.43]

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?

[Richard Burts]

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

[dangal.43]

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?

[Richard Burts]

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

[dangal.43]

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?"

[Richard Burts · Accepted solution]

Dan

VLSM is certainly one of the essential components of classless addressing and subnetting. And summarization usually goes along with that.

Summarization allows us to more efficiently describe what we have in the network and to reduce the size of the routing table.

So yes I would agree with what you said.

HTH

Rick

[dangal.43]

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.

[Richard Burts]

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