i-NET+ - Question

Author

Question

mich805

2001-04-19, 1:48 pm

Hi,

I have a question about a question that is in the Exam Cram book that I have. For those of you who have it, it is number 23 in chapter 16 on page 307. For those of you who don't have it, the question reads:

Which of the following Class B subnet masks will provide for 12 subnets with at least 4,000 hosts per subnet?
A. 255.255.0.0
B. 255.255.192.0
C. 255.255.240.0
D. 255.255.255.0

The book says the answer is C

But when I do the problem out, this solution would allow for 4,096 hosts, but only 6 subnets. This was how I got to that answer:

If the mask is 255.255.240.0, then 4 bits would be borrowed from the second to last octet, leaving 12 bits for the host id. 2 to the power of 12 is 4,098 minus the 2 unusable addresses equals 4,096 hosts per subnet. But 2 to the power of 4 is only 8 minus the 2 unusable addresses equals 6 subnets.

Have I done this incorrectly?

Any feedback would be greatly appreciated.

Thanks,

Mich

SasiSan

2001-04-19, 2:33 pm

The book is right. 2 to the power of 4 is 16 (minus 2 supports 14 subnets.)

I think what you did was counted the number of bits -- 1 - 2 - 4 - 8, and thought four bits so 2^4=8.

I do that alot too. BUT the 1 doesn't count because that is 2^0=1. So 1 - 2 - 4 - 8 - 16 -- 16 would be 2^4.

All your other math was perfect.

mich805

2001-04-19, 3:15 pm

OOOOOHHHH,

THANK YOU!!

I feel so stupid now. It all makes sense. What a careless mistake.

Thanks a bunch!

SasiSan

2001-04-19, 3:20 pm

We all get that feeling... And it was a good question!!! Question everything!!!

Good Luck!!!

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