VLSM, CIDR and IPv6 confusion.

Posted on 2005-04-24
Medium Priority
Last Modified: 2009-08-04
Hi people,

Could someone helps me understand these things, please?

1- what's the different between VLSM and CIDR?
2- If we design IP based on CIDR, Will it be confused to the REAL world that you have duplicate IP address as someone else?
3- What kind of consideration when we want to upgrade from IPv4 to Ipv6?

Thanks a lot experts :)

Question by:sokhodom
LVL 10

Accepted Solution

plemieux72 earned 1200 total points
ID: 13853378
VLSM is used to further subnet your subnets.  It allows creating different size subnets and reduces IP address waste.  
CIDR is kind of the opposite and is also called supernetting or aggregation or summarization.  It allows you to represent many subnets using one address/mask.  This is used on routers to reduce the size of access lists and routing tables and consequently reduce the processing requirements of routers and improves performance.

CIDR can be used with either private IP addressing or public.  In fact, the Internet routers are already configured with CIDR using public addressing.  This certainly doesn't prevent you from using it on your private networks (, or  In fact, if you have a large network, you should plan the addressing with both VLSM AND summarization in mind.

IPv6 migration will require co-existence with IPv4 for a long while through either or all of the following:
Dual-stack (IPv4 and IPv6 on same machine), Translation (between 4 and 6) and Tunneling (from a v6 network to another via a v4 network)
... Here is the main tech page on IPv6 at Cisco for more info:

Author Comment

ID: 13853382
Great explanation !! :)
Thank you,

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