CIDR / VLSM whats the difference?

Unanswered Question
Jul 8th, 2008

I don't see a difference, other than the term CIDR is used for public addresses as VLSM is the term used for RFC1918 addresses.

What am I not seeing here?

I have this problem too.
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Richard Burts Tue, 07/08/2008 - 11:41


I would not agree that VLSM is used for RFC1918 addresses. VLSM applies equally to the private RFC1918 addresses and to public addresses.

I believe that there is much similarity and overlap between the terms CIDR and VLSM. But if you are looking for differences I would suggest that CIDR is more typically used in talking about aggregating addresses and especially aggregating or summarizing addresses beyond their normal boundaries ( would be a CIDR block of addresses which takes addresses that appear to be class C and creating a block with more than 256 addresses). VLSM is more typically used in talking about breaking networks down into subnets where the subnets have differing subnet masks (,, would be an example of VLSM which takes a class B network and creates multiple subnets with different subnet mask lengths).



Route_reflector Thu, 07/10/2008 - 20:30

Hi, I dont agree in CIDR & VLSM Depict the same?

1st CIDR was designed to eliminate the classfull subnet system..

Hence we started to use a same (small) CIDR value for the entire classful nw.

Example: If Class C range is broken with /27, we will get 8 small subnets, Where all are equal subnets.

VLSM states that we can use more than one subnet value in any network

Example: In nw, you can get a /27, /29, /26/, /30 Where you get uneqaul subnets when breaking that range.

Note: CIDR/VLSM do not have any relations with Public/Private Classes.

Hope this is useful to you.



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