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totaramFlag for United States of America

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subnetting

I am having trouble with the following question, even though it looks simple:
A co needs to have min of 300 subnets each with max of 50 hosts per subnet.Working with class B address space, which of the follwoing subnets will support this addressing scheme?
 
A. 255.255.255.0
B. 255.255.255.128
C. 255.255.252.0
D. 255.255.255.224
E. 255.255.255.192
F. 255.255.248.0  
Avatar of Renato Montenegro Rustici
Renato Montenegro Rustici
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That would be option B.
Answer F: would give you 65,533 hosts....
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Renato Montenegro Rustici
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Sorry.... 255.255.0.0 would be 65,533
255.255.248.0 would be 2,048 hosts
About the subnetwoks, it usually wont matter (for the testing purposes). Those type of question will always focus on the idea that you must save ip addresses. So, you must always get closer to the minimum host address needed for your network. If you manage to do that, usually there will last enough addresses to the subnets. In your case, 1024 subnets using option E.
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rmrutice;
You are right about "192 is 2^6 that equals 64" calculations, but how are we getting 300 subnets here.
Also, we get 62 hosts for class c network and not class B network...
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There is only one answer that gives you the first part of the question - 50 hosts on each subnet... It wouldn't matter if it was a Class A or B major subnet.

The answer is E

If the question had mentioned using Class C address space, the answer would be "none of the above"

Key to taking these tests is not to get too wrapped up in the minutiae. Find the answer that fits 3/4 of the question right away and then determine if the rest of the question really matters. If "none of the above" is not an optional answer, then is has to be one of the given choices, so pick the "best" solution out of the given choices, which you've already narrowed down.
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Really the answer is B & E...
Answers B & E are equally good:
B. 255.255.255.128 - 512 Class B Subnets, each with 126 hosts
E. 255.255.255.192 - 1024 Class B Subnets, each with 62 hosts

Wrong Answers:
A. 255.255.255.0 - 256 Class B Subnets, each with 254 hosts
C. 255.255.252.0 -  64 Class B Subnets, each with 1024 hosts
D. 255.255.255.224 -  2048 Class B Subnets, each with 30 hosts
F. 255.255.248.0  -  32 Class B Subnets, each with 2046 hosts
Read http://www.nnk.com.au/index.php?option=com_content&view=article&id=20:subnetting-without-tears&catid=3:articles&Itemid=5
The author of the question can, of course, test you in both sides: the subnet calculation and the host calculation. Using the method I provided, you will manage to make both calculations. As I said, usually, they want to know if you are able to maximize the use of the address space in order to plan a addressing schema of your network using the best practices.

Private addresses are not plentiful, so you must use them wisely.