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VLSM question

Hi;

I found this link http://fengnet.com/book/CCNA%20Portable/ch02lev1sec2.html#ch02fig01 to explain how VLSM works, it shows step by step to find out each subnet with using VLSM. This is a good site so far amongs all others site which explain how VLSM works but I still got lost in Step 2.

In Step 2, the ".64" is picked for Network "A", why it is not ".0"?

The website author used .128/26 to break it into two for network "B", can he use .192/26 to break it instead of .128/26? If not, why?

Jimmy Larsson, CISSP, CEH

It is better to say that "/26" is used to break into 4 networks starting with .0, .64, .128 and .192.

Does this make more sense?

/kvistofta

KANEWONG

ASKER

Yes, it is right, it will break into 4 subnets like this..

192.168.100.0/26
192.168.100.64/26
192.168.100.128/26
192.168.100.192/26

But why it is picking 192.168.100.64/26 for Network "A", not 192.168.100.0/26?

Also, in step 5 for Serial Link, the author go back to use /26 subnet for the router interface, not keep going to use /28 which is calculated in Step 4.

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KANEWONG

ASKER

thanks for point it out. Yes, I looked back the Step 5, the author is using .0/26 to create another subnet for router interface. This is the reason why, he picked .64/26 in step 2 for Network "A"

Because both of .0/26 and .64/26 creates 62 hosts. The author can use .0/26 for Network "A" and .64/26 for router interface either, right?

Jimmy Larsson, CISSP, CEH

In ancient times there were limitations in the routers so that the first subnet could not be used. It hasnt been an issue for the last 10 years but maybe that was the reason not to use the .0 subnet? Besides from that it is just a matter of taste which ones to use, all 4 works equally good today.

/Kvistofta

Jimmy Larsson, CISSP, CEH

No points for me?

/Kvistofta