2 subnetting questions.

These 2 questions are from the exam of Train Signal.
It showed my answers were wrong, but I recalculated and still didn't know why.
Maybe the keys were wrong?
Can I have someone help me or confirm that? Thank you very much!

1.
Your network has been assigned the network number 200.10.10.0. Your network manager has asked you to come up with a subnet mask that will allow for at least 15 subnets. No subnet should ever contain more than 12 hosts, but should contain at least five. Which of the following subnet masks should you use?

Your response was:
/28

The correct answer: /29

2.
Your network has been assigned the network number 134.100.0.0. Your network manager requests that you come up with a subnet mask that allows for at least 500 subnets, but no subnet must be able to hold more than 120 hosts. What is the best subnet mask to use?

Your response was:
/25

The correct answer: /26

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Comments

dynamik

Just glanced at it, but yours look right to me.

Come by tomorrow if you want (8pm cst): http://www.techexams.net/forums/mcsa-mcse-windows-2003-general/43776-anyone-interested-free-virtual-training-me-3.html#post323343

beef1218

dynamik wrote: »

Just glanced at it, but yours look right to me.

Come by tomorrow if you want (8pm cst): http://www.techexams.net/forums/mcsa-mcse-windows-2003-general/43776-anyone-interested-free-virtual-training-me-3.html#post323343

Definitely!
Thank you very much!

mens

beef1218 wrote: »

These 2 questions are from the exam of Train Signal.
It showed my answers were wrong, but I recalculated and still didn't know why.
Maybe the keys were wrong?
Can I have someone help me or confirm that? Thank you very much!

1.
Your network has been assigned the network number 200.10.10.0. Your network manager has asked you to come up with a subnet mask that will allow for at least 15 subnets. No subnet should ever contain more than 12 hosts, but should contain at least five. Which of the following subnet masks should you use?

Your response was:
/28

The correct answer: /29

2.
Your network has been assigned the network number 134.100.0.0. Your network manager requests that you come up with a subnet mask that allows for at least 500 subnets, but no subnet must be able to hold more than 120 hosts. What is the best subnet mask to use?

Your response was:
/25

The correct answer: /26

Hi there!

1.
A /29 mask gives you 6 hosts doesn't it? And a /28 gives you 14 hosts, which violates the max 12 hosts requirement.

2.
Atleast 500 subnets in a class B network needs atleast 9 subnetbits (512). That will leave 7 hostbits and max 126 hosts in each subnet. This violates max 120 hosts.

With at /26 mask you'll have 1024 subnets and max 62 hosts - answers the question pretty much.

beef1218

Hi, mens,
Thank you.
I now understand the first problem.
For the second problem, I misunderstood "no subnet must be able to hold more than 120 hosts". I thought it was equal to "each subnet needs to hold up to 120 hosts".

Thank you very much! I have to read very carefully from now on.

dynamik

I'm sticking by your original answers. If someone tells me they need a maximum of X hosts per subnet, I'd pick the one that would accommodate that many.

If someone tells me "I need a maximum of 12 hosts per subnet," I'm not going to go, here's 6

MachX85

beef1218 wrote: »

These 2 questions are from the exam of Train Signal.
It showed my answers were wrong, but I recalculated and still didn't know why.
Maybe the keys were wrong?
Can I have someone help me or confirm that? Thank you very much!

1.
Your network has been assigned the network number 200.10.10.0. Your network manager has asked you to come up with a subnet mask that will allow for at least 15 subnets. No subnet should ever contain more than 12 hosts, but should contain at least five. Which of the following subnet masks should you use?

Your response was:
/28

The correct answer: /29

2.
Your network has been assigned the network number 134.100.0.0. Your network manager requests that you come up with a subnet mask that allows for at least 500 subnets, but no subnet must be able to hold more than 120 hosts. What is the best subnet mask to use?

Your response was:
/25

The correct answer: /26

Question 1 requires 3 bits. 2^3=8 -2 gives you 6 hosts. you need at least 5 so that works. If you used 4 bits you would have 16-2 hosts which exceeds the 12 host limit.
That leaves you 5 subnet bits. class C has 24 network bits, add the 5 subnet bits to get
/29

question 2 says you cant go over 120 hosts per subnet. 7 bits would get you 128-2 hps so you'll have to use 6 bits. 16 network bits are reserved for the class B network, 6 bits for hosts. That leaves 10 subnet bits, add that to the 16 network bits to get a
/26

beef1218

MachX85, thank you for your explanation.
dynamik, that's right, here's 6.