# valid subnet numbers

• posted

The Cisco ccna book has a test question : what are the valid subnet numbers for

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 ) ...

What am i doing wrong ? Any help ?

• posted

I owuld say that The ccna guide tells me the right answer is that subnet 0 is 180.1.0.0 and

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.

• posted

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. Valid Hosts Addresses: 180.1.0.1 - 180.1.7.254

Then next SUBNET starts...

Good luck,

Mike

• posted

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.

• posted

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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