valid subnet numbers

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The Cisco ccna book has a test question :
what are the valid subnet numbers for
180.1.0.0 mask 255.255.248.0
Now...

when I use the formula in my study guide AND when I use a subnet calc
program on the web I get the same
wrong answer, that is valid numbers  in this net are from 180.1.0.1 -
180.1.7.254

The ccna guide tells me the right answer is that subnet 0 is 180.1.0.0 and
then 180.1.8.0 ,180.1.16.0 etc up to 180.1.240.0 (last valid subnet ) ...
180.1.248.0 (broadcast)

What am i doing wrong ?  Any help ?








Re: valid subnet numbers
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I owuld say that
The ccna guide tells me the right answer is that subnet 0 is 180.1.0.0
and
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is the right answer-I cant see how 180.1.7.254 could be the last
available host
The mask is telling you that the subnets are in sets of 8-not that the
range only allows you to go to 180.1.7.254-that is signifying one
possible subnet-the first.
I would be grateful for some one to clarify please.


Re: valid subnet numbers
You have to read question very carefully. If it asks for subnets (network
numbers), then their answer is correct. Networks are: 180.1.0.0, 180.1.8.0,
180.1.16.0, etc. If they ask about HOSTS, then your calculations are right.
Just remember, what components do you have in the subnet:

1. Network Address:    180.1.0.0
2. Valid Hosts Addresses: 180.1.0.1 - 180.1.7.254
3. Network Broadcast Address: 180.1.7.255

Then next SUBNET starts...

Good luck,

Mike
------
Headset Adapters for Cisco IP Phones
www.ciscoheadsetadapter.com


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Re: valid subnet numbers

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It's not asking you for hosts, it's asking you for subnets, "what are the
valid subnet numbers for". The book answer is 100% correct. Thats one of the
BIG gotcha's in a lot of the tests, you need to make sure you read the
question and know exactly what they are looking for.



Re: valid subnet numbers

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Like everyone else says, the question is asking only for subnet (network)
numbers, not valid host addresses in a subnet.

Shortcut I use is take the interesting octet (3rd) of (the .248) in the mask
and subtract from 256.  256-248=8 so blocksize of subnets is 8 (they
increment by 8 in the 3rd octet).
Theoretical valid subnets are 180.1.0.0; 180.1.8.0; 180.1.16.0; 180.1.24.0;
180.1.32.0... and on up to 180.1.248.0.  I believe for the CCNA you can't
use the first or last subnets so practical subnets would be 180.1.8.0;
180.1.16.0; 180.24.0... 180.1.240.0.



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