What is the difference between CIDR and VLSM???

[David]

Hi all Cisco Gurus,

What is the difference between CIDR and VLSM? And what is "supernettng"?

[Martin Cooper]

David <cisc...@hotmail.com> wrote:

> What is the difference between CIDR and VLSM?

CIDR (== Classless Inter Domain Routing) is basically the
abolition of classful routing. Instead of having to assign
address space out of three possible fixed size blocks - a
class A (too big for all but the largest organisations),
a class B (still too big for most organisations, but also
fairly scarce relative to class Cs), and a class C (which
is too small for most organisations) CIDR allows a block
of address space to be defined by a starting point in the
global address space (a prefix), and a size (prefix length).
This has the advantage of increasing flexibility and making
use of parts of the address space that would otherwise have
been wasted.

> And what is "supernettng"?

Supernetting is a CIDR concept. In the old classful days,
if you had subnetted your address space, it was required
to have an explicit route for all your subnets, as the
default route was only used for addresses not in your
routing table at all - traffic to a subnet without a
route would be dropped. With supernetting, traffic is
instead passed to a "less specific" route (often the
default) which is useful because it allows subnet
routers to rely on core routers upstream to have
more specific routing information.

VLSM (== Variable Length Subnet Masks) is an extension of
CIDR that makes it possible to subnet a globally assigned
block of address space using different masks, so as to be
able to have different size subnets within the same block.
In the old classful days, it was necessary to use one and
only one subnet mask per address block, so all subnets had
to be the same size.

Hope that helps!

Cheers,

Martin

[Seeker]

The simple version that I like to think of are the following.

VLSM is subnetting version 2.0
You are still breaking up networks into smaller parts, but with VLSM
you can break that one network into unequal pieces. This really helps
to stretch those avaliable IP's. The format that I have seen most
used is to keep the larger subnets at the beginning of the range and
the smaller subnets towards the end.

CIDR or supernetting is basically the opposite of subnetting.
Instead of breaking up networks, you are combining them together by
adjusting the subnet mask associated with your routes. This is a good
technique for reducing the size of your routing tables, but becareful
to only include the routes which are on that router. It is easy to
over advertise and cause loops on a network.

Hope this helps.

[Jeff]

If you're looking for a good reference that explains all this, and gives you
practice exercises to drill it into your head, do a search for the white
paper on 3Com's website.

Jeff

[zher]

Martin Cooper's explanation is very detailed.
In one word, CIDR is for supernetworing, VLSM is for subnetworking.

"David" <cisc...@hotmail.com> wrote in message news:<9l88ne$8t...@imsp212.netvigator.com>...

[Mr. Robyn Myers]

I usually always say, CIDR is one you buy and VLSM is homemade. In other
words, when you order an IP block, it is CIDR. You can take a block and add
VLSM to it.

Robyn

"zher" <zhe...@yahoo.com> wrote in message
news:ca424915.0108...@posting.google.com...

[iamthe.a...@gmail.com]

despite the takeover by HP, this still exsits:
https://www.coursehero.com/file/10825394/IP-address-and-subnetting/
enjoy!