valid subnet numbers

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barret bonden's Avatar (by Gravatar) posted on
February 28, 2007, 8:34 pm
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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 ?








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daytime's Avatar (by Gravatar) posted on
February 28, 2007, 10:58 pm
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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.



 
headsetadapter.com's Avatar (by Gravatar) posted on
February 28, 2007, 11:07 pm
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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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Brian V's Avatar (by Gravatar) posted on
February 28, 2007, 11:52 pm
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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.




 
Cisco Kid's Avatar (by Gravatar) posted on
March 1, 2007, 2:51 am
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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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