Question : variable lenght subnet mask

Hi,

What is VLSM?  Could anyone pls. tell how I can do VLSM with some examples( with explanation)?  what is the difference between supernettng and VLSM when compare to subnetting?

ayha1999.

Answer : variable lenght subnet mask

Sciwriter, y our typical cable/dsl router is not the place to be worrrying about VLSM and route summarization - "supernetting".

Supernetting is taking the same process I mentioned above and reversing it.  If you have those same 8 subnets, for instance, you could ask yourself ... "self, how could I write one line that means the same thing as these 8 networks?"

Here's the networks:

172.16.0.0
172.16.32.0
172.16.64.0
172.16.96.0
172.16.128.0
172.16.160.0
172.16.192.0
172.16.224.0

So, without resorting to binary (the "fast" and easy way), let's just walk the bits back one at a time.  Our networks above are /19's.  Ok, let's say they're /18.  Now what?
A /18 would result in 1/2 as many networks - right?  So, instead of 8, we'd have 4 (2^2).  We know that 256-194 (.194 is the mask - 2 bits) = 64.  So, the networks must increment by 64.  This results in:

172.16.0.0
172.16.64.0
172.16.128.0
172.16.192.0

This is not exactly what we wanted, but it does illustrate that if we had wanted to summarize .32 and .0 for instance, you could simply say 172.16.0.0/23.

Ok, let's keep going - if we move the bits to /17 that's 1/2 as many again, now leaving 2 networks. (2^1).   One borrowed bit is .128 (1000000) so that means the networks must increment by 256-128=128.

172.16.0.0
172.16.128.0

So, 4 networks (from 0-128) could be summarized with 172.16.0.0/17

Now, for the final summarization, using the same procedures as before - 1/2 as many =1.  Bit mask is now /16.  No borrowed bits 2^0.

172.16.0.0/16

There's a way to do this in binary for numbers which are not so obvious - but it's too early in the morning for that ... :)