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## Subnetting doubt

I am pretty confident of how to do the subnetting of any paticular class but what i am doubtful about is the concept, when you chose certain number of bits from the host portion in order to be used as subnet bits. Do we use the formula

[b]2 to the power of "n" - 2 [/b] or do we use the formula [b]2 to the power of "n"[/b] .

As i have read it in a number of places that when we put in the 'ip subnet-zero' command on the router we can use the second formula instead of having a minus 2 in our formula but Cisco itself in the CCENT/CCNA ICND1 book says that we use the first formula in order to calculate the number of subnets..

Now can anyone confirm it whether we use the first formula or do we use the second formula when studying for the CCNA examination. Also if you have any link of the CISCO website that says so, it would be great..

7 REPLIES
Green

## Re: Subnetting doubt

Generally speaking, for the sake of examination, the questions are worded in terms like "What is the total number of subnet {blah blah blah}" or What is the total number of USABLE subnets {blah blah blah} ... There are other typical clues presented as well.

If you understand the concepts and processes, there will be no real doubt in your mind about which the question is asking for.

Good Luck

Scott

Community Member

## Re: Subnetting doubt

If ip subnet zero is disabled on the router use:

1. Max # of subnets = 2^n - 2 (n = no. of masked bits in subnet mask)

2. Max # of host per subnet = 2^n - 2 (n= no. of unmasked bits in subnet mask)

If IP subnet zeo is enabled on the router use

1. Max # of subnets = 2^n

2. Max # of host per subnet = 2^n - 2

Note, only the first formula you change.

Community Member

## Re: Subnetting doubt

I'm reading an Exam Cram II book for INCD1. It says in one paragraph, "The rule is known as Subnet Zero Rule, and obviously if it is in effect, we lose 2 subnets from the total we create."

Next paragraph says, "The command IP SUBNET ZERO turns on the ability to use them, so that might be how the question is telling whether they are in effect."

No, it's not obvious at all, not to me at least.

"if it is in effect, we lose 2 subnets from the total we create" Tell me that IP-SUBNET ZERO forces me to subtract 2 subnets.

"The command IP SUBNET ZERO turns on the ability to use them..." This tells me that I can keep 2 subnets per network.

Which is it? According to rynex1234, if it is in effect, we do not subtract. Correct?

Thanks,

Vince

Community Member

## Re: Subnetting doubt

What is the default for the exam? Is it off or NO IP-SUBNET ZERO? In other words, if it is not mentioned, I'm assuming NO IP SUBNET ZERO.

Community Member

## Re: Subnetting doubt

What is the default for the exam? Is it off or NO IP-SUBNET ZERO? In other words, if it is not mentioned, I'm assuming NO IP SUBNET ZERO.

Community Member

## Re: Subnetting doubt

What is the default for the exam? Is it off or NO IP-SUBNET ZERO? In other words, if it is not mentioned, I'm assuming NO IP SUBNET ZERO.

Hall of Fame Super Gold

## Re: Subnetting doubt

Vince

The default in IOS is to enable subnet zero. It has been the default for a LONG time. I would assume that if the exam cares about it that the exam will indicate whether subnet zero is active or not. I would assume if there is no indication about it that the default for the exam is subnet zero activated.

And as a technical detail, the operation of subnet zero only impacted use of a single subnet (not two subnets as is frequently stated).

HTH

Rick

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