CIDR block question

Hi there.
Im studying for my ccna and all seems to be ok.
Im just after finishing learning about subnets and VLSM's.
Im answering the review questions in my book and have come
across a question I simply cannot answer correctly.
I hope someone can help me, here's the question:

Which of the following IP addresses fall into the CIDR block of
110.68.4.0/18? (Choose three).

A. 110.68.8.32
B. 110.68.7.64
C. 110.67.6.255
D. 110.66.3.254
E. 110.65.5.128
F. 110.64.12.128


These are class A adresses for a start.
The subnet mask is 255.255.192.0.
The block size for the third octet is 64.
So, the subnets for the second octet is 0, 1, 2, 3, 4...255
and the subnets for the third octet are 0, 64, 128, 192.
The answer in the book states B, C and E.
I would have said that the above IP address in the question is within the 110.68.0 subnet.
I find this question a little vague and Im unsure how to answer it.
Thanks for your help and best regards!

The only correct answers I see here are A and B. As you said it should be 110.68.x.x. The valid subnet range (block) is from: 110.68.0.1 to 110.68.63.254 (not counting the first and last address). Any thing else is out of the bounds. Thats as far as I know.

Hi there and thanks for the reply.
I have a feeling the answers in the book pertaining to this
question are wrong, strange.
Thanks again though!

I would have to agree with A & B

Your book is incorrect. YOu need to take that book back and get your refund.

110.68.4.0/18 is part of 110.68.0.0 to 110.68.63.254. Therefore, the subnetwork at 110.68.0.0/18 includes the IP address of 110.68.4.0. Answers C, D, E, and F all fall outside of this range.

Because your subnetwork prefix is greater than /16, which includes the first and second octect, this can only mean that in order for the answer to fall within the range, the first two octet of the answer must be 110.68.

Hi there.
Im studying for my ccna and all seems to be ok.
Im just after finishing learning about subnets and VLSM's.
Im answering the review questions in my book and have come
across a question I simply cannot answer correctly.
I hope someone can help me, here's the question:

Which of the following IP addresses fall into the CIDR block of
110.68.4.0/18? (Choose three).

A. 110.68.8.32
B. 110.68.7.64
C. 110.67.6.255
D. 110.66.3.254
E. 110.65.5.128
F. 110.64.12.128


These are class A adresses for a start.
The subnet mask is 255.255.192.0.
The block size for the third octet is 64.
So, the subnets for the second octet is 0, 1, 2, 3, 4...255
and the subnets for the third octet are 0, 64, 128, 192.
The answer in the book states B, C and E.
I would have said that the above IP address in the question is within the 110.68.0 subnet.
I find this question a little vague and Im unsure how to answer it.
Thanks for your help and best regards!

Thanks again for the informative replies lads.
It's cleared things up for me. Maybe I will pass
my CCNA afterall!


Regards[/i][/b]