subnetting question..that is bugging

Unanswered Question
Jun 6th, 2007

See this question? If mask were used with a Class B network, how many subnets could

exist, with how many hosts per subnet, respectively?

Answer: E. Class B networks imply 16 network bits, and the mask implies 7 host bits (7

binary 0s in the mask), leaving 9 subnet bits. 29 ? 2 yields 510 subnets, and 27 ? 2 yields

126 hosts per subnet

I would have said the answer is 512 subnets and 126 hosts.

Sometimes the rule is to minus 2 from the subnets?hence 510 and sometimes it?s not???.hence 512.

Do you know when you minus 2 from subnet?s and when you don?t??

Does it have something to do with if subnet zero is defined?

I have this problem too.
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thefindjack Wed, 06/06/2007 - 11:17

You only have to take out the first and last subnet when the "ip subnet-zero" command is not setup on your router. The command is there by default on all IOS.

dentaquest Thu, 06/07/2007 - 03:49

this is the conclusion I was coming to...and what I have found is there is a heck of a lot of training materials out there..that ask you questions..but dont specify if subnet zero is on or the ciso press book that showed me this question..I bought it recently..and it's for 640-801...does anyone know the default on the ccna subnet zero on or off...thanks.

Wilson Samuel Thu, 06/07/2007 - 07:14


By default apply this thought whenever you are answering in the exam:-

1. If its MCQ option choose the correct answer whichever is specified in the options

2. If both the options are given then use the option with Subnet Zero

3. If its 'Fill the Blanks', put the Subnet Zero value.

I hope this helps, please rate if it does help.

Kind Regards,

Wilson Samuel

mohammedmahmoud Thu, 06/07/2007 - 11:56


I agree on Wilson's suggestion for the exam, but i've one clarification, to be able to use the all 0s subnet on a router you must have "ip subnet-zero" - while the all 1s subnet is used by default with no need for this command - As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, which is most most most probably the case on your exam questions.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

michael.balogh Mon, 06/11/2007 - 10:08

Cisco has allowed use of the 0 and 255 subnets on their exams for over a year now.


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