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difference between CIDR and VLSM

Dear Friends,

can u give the exact and illustrative difference between CIDR and VLSM




Re: difference between CIDR and VLSM

VLSM is a way to subnet (break the existing network) into smaller networks. This way you can save ip addresses, assigned to various networks, with specific host number requirements. For example. you could assign a mask of /30 rather than /24, for a wan link which requires only 2 host ip addresses. /30 would be the right mask for this network, with 2 hosts, where as if you use /24, you lose 252 addresses which cannot be reused anywhere else on that network.

CIDR on the other hand is used for summarization. THis is basically used by ISPs, to reduce the number of routes in the routing table. IF an ISP has a big block of address say from the network, whose subnet has been assigned to its various customers, say to customer A, to customer B and so on, the ISP can summarize all these networks into one single route with a mask of /16 ( and advertise this route to other ISP routers on the internet. This reduces a lot of routing table size (which obviously consumes lot of memory on the router).

Hope that helps a bit!