Sybex unclear question

TraiCauVongTraiCauVong Member Posts: 5 ■□□□□□□□□□
in CCNA & CCENT
Hi everyone. I'm reading Sybex and there's something I want to ask someone who knows:
11. 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
The answer is:
11. B, C, E. A Class A network address with a /18 is 255.255.192.0. The subnets in the third octet
are 0, 64, 128, 192. The network address in the question is 110.64.0.0, with a broadcast of
110.64.127.255, since the next subnet is 110.64.128.0. Answers B, C, and E are correct host IDs.
My answer was A and B ( I could not find the third possible solution here).
So I was wrong or the book was wrong.
Please help, I'm newbie here and newbie in CCNA as well.
Thanks friends![/b]
Be brave, move on!!!
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Comments

  • mikearamamikearama Member Posts: 749
    That is one messed up question. I believe the book is entirely wrong.

    A network address of 110.68.4.0/18, though a class A, is subnetted into the third octect. So no host ID can use the first or second octects at all. That your book thinks answers C and E are possible is crazy (since they require stealing back network bits for hosts).

    So, using 255.255.192.0 as the mask, the usable subnets are:
    110.68.0.0
    110.68.64.0
    110.68.128.0
    and 110.68.192.0

    This mask allows for 2**14 - 2 hosts... totalling 16382... per subnet.

    Since the question provides 110.68.4.0 as the working subnet, we know we're in the first available working subnet, 110.68.0.0, and can go as far as 110.68.63.254.

    The answers that fit are, as you suggested, A and B. There are no other options in the list provided.

    Again, a messed up question. The only way the answers work that were given in the book is if the mask was /10, not /18. Where did you get this question from?

    Mike
    There are only 10 kinds of people... those who understand binary, and those that don't.

    CCIE Studies: Written passed: Jan 21/12 Lab Prep: Hours reading: 385. Hours labbing: 110

    Taking a time-out to add the CCVP. Capitalizing on a current IPT pilot project.
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  • TraiCauVongTraiCauVong Member Posts: 5 ■□□□□□□□□□
    Thanks Mike. It's from Sybex Edition V E-book that I downloaded from gigapedia.com. It's a free stuff so no one can guarantee its quality, right! :D:D:D.
    Thanks again Mike!
    Be brave, move on!!!
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  • r_durantr_durant Member Posts: 486 ■■■□□□□□□□
    Hi Guys,

    Here is the errata from the Sybex site for this question...the last one is what it should be

    Page 152

    Chapter 3 Question 11 - Updated 04/20/05
    Support, the new question and explanation is after the old one - the
    question, answer and explanations were wrong somehow. Please replace this
    question in the next printing:

    ____________________________________________________________
    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.
    New explanation:
    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.



    *NEW*
    This is really an issue with your revised solution to an errata
    problem already reported, regarding Question 11 in Chapter 3, pages
    152 & 155. Your revised answer reads:

    *NEW*
    ____________________________________________________________
    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.
    New explanation:
    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.

    Page 155

    Chapter 3 Question 11 - Answer
    Support, the new question and explanation is after the old one - the
    question, answer and explanations were wrong somehow. Please replace this
    question in the next printing:

    ____________________________________________________________
    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

    Answer: B, C, E.

    A Class A network address with a /18 is 255.255.192.0. The subnets in the
    third octet are 0, 64, 128, 192. The network address in the question is
    110.64.0.0, with a broadcast of 110.64.127.255, since the next subnet is
    110.64.128.0. Answers B, C, and E are correct host IDs.



    *NEW*
    ____________________________________________________________
    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.
    New explanation:
    A Class A network address with a /22 is 255.255.255.252. The subnets in the
    fourth 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.
    CCNA (Expired...), MCSE, CWNA, BSc Computer Science
    Working on renewing CCNA!
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