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CIDR and VLSM

ravi saxena
Contributor

CIDR and VLSM

Hi,
Can someone help me in telling the exact difference between CIDR and VLSM.
regards
Ravi
1 REPLY
Ardon
Trusted Contributor

Re: CIDR and VLSM

Hi Ravi,

It's a thin line between the two. Basicaly both take a shorter Network Prefix as would be expected based on the natural IP Classes.
So 10.10.10.0/4 as an example. To cut a long story short, CIDR is used in the Internet while VLSM is used within an organisation (Cisco defenition).
Now why do that? This way you aggregate routes into one thereby greatly reducing the size of the Routing Table.
Secondly, and as much as important is the fact that you know can use different length Subnet Masks throughout the network (when using a Clasless Routing Protocol like RIPv2, OSPF) which sends the used Subnet Mask along it's routing updates. When using a Routing Protocol that does NOT send the Subnet Maks along route updates (like RIPv1)you NEED a constant Subnet Mask throughout the WHOLE network. Now no big issue but....if you just need a link between two routers and are using a /24 network that would mean you have to assign a /24 network (254 Host IP-2=252 Host IP WASTED.

Hope this helps, Ardon
ProCurve Networking Engineer