VLSM Confusion

jwills

Can someone explain to me what Odom means when he says, A COMMON MISTAKE OCCURS WHEN PEOPLE THINK THAT VLSM MEANS “USING MORE THAN ONE MASK,” RATHER THAN “USING MORE THAN ONE MASK IN A SINGLE CLASSFUL NETWORK.”

A couple of example would help.

Thanks

notgoing2fail

It just means that all your networks don't have to have the same subnet mask.

192.168.1.1 /27


As long as your masks don't overlap, you can have /27 /24 /26 whatever, however you want to work out your design....

Your routing protocol must support subnet masks in their advertisement, this is what makes them classless routing protocols.... RIPv2 for example...

jwills

That part i understand. What i'm confused about is he says vlsm is used in a single classful network. What specifies what a single classful network is?

He is stating that 10.0.0.0 and 11.0.0.0 are two different single classful networks. But he is also implying that 10.3.4.0 and 10.3.5.0 are in the same single classful network.

I guess theres a difference between 2 different subnets and 2 different classful networks.

Any input would be of help.

chmorin

Classful address fall under class A, B, and C. Generally 10. address. 172.16. address, and 192.168.1. addresses. VLSM does not fall under those characteristics.

For vlsm you can have two (or many) networks fall under the same classful grouping.

It is generally not seen as this though, since vlsm is "classless" so saying it has anything to do with classes is kind of bad and will only confuse you.

EDIT:

What odom is trying to say is I can have a 192.168.1.0 /26 network. Which falls inside the 192.168.1.0 /24 class C network range.

jwills

Thats kinda how i was understanding it.

Thanks for your help.