Another subnetting question

For all you subnetting warriors out there, the following question is a known errata on the Sybex 5th edition CCNA study guide. Basically the initial answer that was given is incorrect and it doesn't match the actual question. Though the correct answer was given on the correct question, nevertheless I wanted to answer this question as if it was not an error:

Which of the following IP addresses fall into the CIDR block of 110.68.4.0/18?

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

I already have my choice(s) of what I believe is the right answer but I wanted to see what others get. Anyone want to take a stab?

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Comments

Pash

Explain to us how you got your CIDR block. What do you look for.

dtlokee

I think they have the mask wrong because with a /18 the range would be 110.68.0.0 - 110.68.63.255 and there are multiple answers in that block. I'm guessing since they used 110.68.4.0 as the "subnet" address they wanted a mask of /22 not /18 in which case the correct answer would be "B" because the range would be 110.68.4.0 - 110.68.7.255 with a /22.

aragoen_celtdra

Pash wrote:

Explain to us how you got your CIDR block. What do you look for.

You ARE gonna make me work for it, aren't you?

This is how I worked it out:

First I looked at the subnet mask/prefix, which is 255.255.192.0

So that tells me the subnet is 110.68.0.0. Then to solve for the 3rd octet, I basically did what Wendell Odom told me in his book, 256-192=64.

So i get 110.68.0.0 and the next subnet is 110.68.64.0

So the range of valid IP address is 110.68.0.1 - 110.68.63.254. So based on that range, that's how I got my answer from the choices given.

Is my method right?

aragoen_celtdra

dtlokee wrote:

I think they have the mask wrong because with a /18 the range would be 110.68.0.0 - 110.68.63.255 and there are multiple answers in that block. I'm guessing since they used 110.68.4.0 as the "subnet" address they wanted a mask of /22 not /18 in which case the correct answer would be "B" because the range would be 110.68.4.0 - 110.68.7.255 with a /22.

You're absolutely right, the mask given was wrong. And I wouldn't have a clue how you deduced that the right mask is /22 (which you are correct again). And you are also correct on the range for the /22 mask. Although the set of answers on the revised question is completely different.

Here is the book's correction:

Which of the following IP addresses fall into the CIDR block of 110.68.4.0/22? (Choose three.)

A. 110.68.8.32
B. 110.68.7.64
C. 110.68.6.255
D. 110.68.3.254
E. 110.68.5.128
F. 110.68.12.128

Answer: B, C, E.
A Class A network address with a /22 is 255.255.252.0. The subnets in the third octet are 0, 4, 8, 12, etc. The network address in the question is 110.68.4.0, with a broadcast of 110.68.7.255, since the next subnet is 110.68.8.0. Answers B, C, and E are correct host IDs.

But with the /18 mask, I did get 110.68.0.0 - 110.68.63.255 as my range. But are you saying that the way the question was posed is incorrect? I thought it was only the answer that was wrong. Clearly I still have a ways to go before I can get a solid grasp of subnetting.

But thanks to everyone for at least making it bearable and fun for me.;)

dynamik

aragoen_celtdra wrote:

You're absolutely right, the mask given was wrong. And I wouldn't have a clue how you deduced that the right mask is /22 (which you are correct again). And you are also correct on the range for the /22 mask.

You need 22 bits to get to 110.68.4.0: 01010010.01000100.000001hh.hhhhhhhh

(which leaves you with 10 host bits)

Pash

aragoen_celtdra wrote:

But with the /18 mask, I did get 110.68.0.0 - 110.68.63.255 as my range. But are you saying that the way the question was posed is incorrect? I thought it was only the answer that was wrong. Clearly I still have a ways to go before I can get a solid grasp of subnetting.

But thanks to everyone for at least making it bearable and fun for me.;)

You are closer than you think. Your method is good just keep practicing. Subnetting is fun when you can do it so quickly you don't even think of the binary methods.

I am still the quickest at work with subnetting and thats against 10-15 year IT vets, the CCNA drills it into you

aragoen_celtdra

dynamik wrote:

You need 22 bits to get to 110.68.4.0: 01010010.01000100.000001hh.hhhhhhhh

(which leaves you with 10 host bits)

I knew that but I didn't know that.

Makes sense?

Pash wrote:

You are closer than you think. Your method is good just keep practicing. Subnetting is fun when you can do it so quickly you don't even think of the binary methods.

I am still the quickest at work with subnetting and thats against 10-15 year IT vets, the CCNA drills it into you

Appreciate the vote of confidence. Hehe! it'd be cool to have a nice subnetting competition at work.

For some reason I have this urging that I need to know subnetting inside and out. Not just enough to pass the test but to actually be able to discuss it with someone else in a very intelligible manner.

My wife says there's something sexy about the way I talk to her about the 0s and 1s and how they all fit together, regardless of whether she actually pays attention to what I'm saying or not.

I guess chicks do dig our inner geeks.

So, careful with the 1s and 0s at work, lest you become ridiculously attractive to the female colleagues.

phantasm

aragoen_celtdra wrote:

My wife says there's something sexy about the way I talk to her about the 0s and 1s and how they all fit together, regardless of whether she actually pays attention to what I'm saying or not. I guess chicks do dig our inner geeks. So, careful with the 1s and 0s at work, lest you become ridiculously attractive to the female colleagues.

My wife is a scientist and loves my geeky side. lol. She gets glassy eyed when I start talking Cisco, when she starts talking Biology, I get dumbfounded. I don't understand a damn thing she says. rofl.