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VLSM question

Posted on 2010-08-23
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Last Modified: 2012-05-10
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?

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Question by:KANEWONG
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by:Jimmy Larsson, CISSP, CEH
ID: 33503386
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by:KANEWONG
ID: 33503498
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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rfc1180 earned 1000 total points
ID: 33503576
>In Step 2, the ".64"  is picked for Network "A", why it is not ".0"?
He could have used .0/26, but he chose not to and decided to use .0/26 to determine network numbers for serial Links.

>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?
Yes he could have used the .192, it is a matter of preference of which one to use.
I typically leave the last (Few of the last subnets alone for growth)

Billy
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by:KANEWONG
ID: 33503635
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?
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by:Jimmy Larsson, CISSP, CEH
ID: 33503642
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
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by:Jimmy Larsson, CISSP, CEH
ID: 33512041
No points for me?

/Kvistofta
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