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Subnetting VLSM

Last response: in Networking
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May 2, 2011 3:23:15 PM

Hello everyone, my name is Yoshiki and I have a problem with subnetting.
I am taking cisco course and i'm on the routing topic.
I've been given a practice paper with answers but I'm stuck with it.
The scenario is:
I have been given the address
192.100.10.0 /23
The total number of hosts are 408;
I have to figure the subnet address and subnet mask.
I worked out the subnet mask easily but the addressing is concerning me because I have these addresses: (these are answers)

1|192.100.10.128 - 90 hosts
2|192.100.11.0
3|192.100.11.224
4|192.100.11.228
5|192.100.11.192
6|192.100.11.64
7|192.100.11.228
8|192.100.10.0 -100 hosts
9|192.100.11.128
10|192.100.11.224

Could somebody tell me why 1 and 8 are still on 192.100.10 and why the rest change to .11? I think it has to do with the number of hosts

thanks for you help, I'd apprectice any answer and for someone to point me the correct topic to read .
p.s Sorry for the english

More about : subnetting vlsm

May 3, 2011 4:10:56 AM

Try looking at it from a binary standpoint. You have 4 octets (32 bits) and the first 23 bits are always the same... and you can't change those. The remaining 9 bits are yours to address at your discretion. So here's what it looks like:

192 = 11111111
100 = 11111111
10 = 1111111(0 or 1 for the remaining bit)
0 = 00000000

The reason some of the networks are .10 and some are .11, is because the last bit of the 3rd octet (the 1 bit) can either be a 1 or 0. If it's a 0, it's the .10 network. If it's a 1, it's the .11 network.

In your scenario, the two .10 networks are using .128 subnet masks, the 0-127 network and the 128-255 network.

Does this make sense?
!