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VLSM & CIDR

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Vishal Sinha

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Jan 20, 2000, 3:00:00 AM1/20/00
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Hi,

Is there any difference between CIDR and VLSM?

Thanks,

Vishal

James Carlson

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Jan 21, 2000, 3:00:00 AM1/21/00
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"Vishal Sinha" <vsi...@fore.com> writes:
> Is there any difference between CIDR and VLSM?

Yes; a very narrow mostly semantic one. CIDR means that you carry a
network mask with every IP prefix. You can no longer guess at the
network mask based on the address range. This allows you to do things
like supernetting class C allocations. VLSM means that within a
single network you can support networks with different subnet mask
lengths. In obsolete protocols like RIPv1, all subnets within a
network were assumed to have the same subnet mask. (Of course, the
current RIPv2 protocol does not have that problem.) VLSM requires
essentially the same software support as CIDR.

(I suppose you could have a broken implementation that carries the
network mask as necessary, but always ors-in the classful network mask
on every subnet mask. You might claim that this hypothetical botch
supported VLSM but not CIDR.)

--
James Carlson, System Architect <car...@ibnets.com>
IronBridge Networks / 55 Hayden Avenue 71.246W Vox: +1 781 372 8132
Lexington MA 02421-7996 / USA 42.423N Fax: +1 781 372 8090
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp

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