calculating usable subnets

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Feb 20th, 2010
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Can someone please explain to me an inconsistency in the ICND1 Lab Guide concerning the calculation of usable subnets? I emphasize “usable” since Cisco does.

Lab 4-3, Task 4 states that we are assigned the address space /28. Q2 asks how many subnets result using this mask/prefix. The answer given is 16, or 2 to the 4th power. Now look at Task 5:, number of subnets = 126, or  2 to the 7th power minus 2. So why “minus 2” here and not in Task 4? Same applies to Task 6:, number of subnets = 510, or  2 to the 9th power minus 2.

I learned that the zero subnet is not allowed in theory, unless explicitly configured to be allowed in praxis in a working network; that the ‘broadcast’ (last mathematically possible) subnet is never allowed; and therefore to assume “2 to the nth power minus 2” (or “minus one” if the zero-subnet is explicitly to be allowed).

This inconsistency in the documentation has serious consequences for those taking the CCNA exam. What am I to tell my students? Is the formula “2 to the nth power minus 2” correct, or is it “ 2 to the nth power”? TestKing is no help on this either, as their exam prep simulator also indiscriminately uses both formulas, sometimes allowing, sometimes disallowing, both the zero and ‘broadcast’ subnets.

Flash Weldon

CCSI 21012

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Leo Laohoo Sat, 02/20/2010 - 03:44
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TYLER WEST Mon, 04/26/2010 - 08:17
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It is an annoying discrepancy.  I'm not an actual trainer, as it appears you are, but have spent quite a bit of time tutoring my coworkers and others on the art of subnet masking and calculations helping them prepare for their tests.  So the question that is looking for the total number of subnets without any indication of usability SHOULD NOT subtract the 2.  That is really how many subnets there are.  When usability comes into play though, it gets gray depending on the implementation.  A few years ago it would be clear because you just "followed the rules" and didn't use the first or last one.  Now, most of the equipment will allow for either.

For a matter of teaching my "students" I instruct them to know both.  You have to figure the total before you can figure the "usable" by subtracting 2.  I have rarely seen a question that would include both 16 and 14 as possible answers or 128 and 126.  You determine both and it has to be one or the other.  In the event that both do appear as possible answers then look carefully at the context of the question and determine if they are looking for the total number of subnets or the number of usable subnets.

Tyler West, CCNP

CWI, Inc.

Flash Weldon Mon, 04/26/2010 - 12:30
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Thanks for your reply. I have found that as far as the exam is concerned, the crucial piece of information is whether the CLI command "ip subnet zero" is on or off. If the function is on, then usable subnets equals total subnets; if off, then usable subnets means total minus two.

Know both. Good advice.



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