PLEASE HELP!!!!!

PacketFilter

Hello everyone - I've been studying for the CCNA exam but I'm a little confused about the correct way to get the number of subnets. All the books that I've been reading have the following formula: 2 to X power - 2. X = # of bits turned on. Others have been saying the NEW way to calculate it is just 2 to X power; no need to substract the 2. Please help me as far as the CCNA.

Thank you in advance.

shednik

the difference is depending on whether or not ip subnet zero is on or off...if it is on you do not subtract 2, if it is not on then you do subtract the 2...

PacketFilter

THANKS!

Netstudent

You should assume that it is on because it is on by default with a modern IOS. The test will tell you otherwise if they want you to calculate the asnwer with ip subnet-zero turned off. I don't think that both possibilities will be in the same asnwer choice anyways.

mrj

I'm quite sure the CCNA (at least 801 version) is 100% NO ip-subnet zero, thus -2.

The difference is - in older IOS systems you COULD NOT use the subnet of, say, 192.168.0.x - It had to be at a minimum, 192.168.1.x.

In modern IOS subnet zero is common and you'll kick yourself in the head when you finish the CCNA because you'll be stuck in no-subnet zero land, which is a myth nowadays. But for the 801 there is NO subnet zero.

If your questions tell you specifically that ip-subnet zero is on or off, what I explained above is the difference.

techster79

They will specify if ip subnet zero is used or tell you about a classless routing protocol(hint hint) which you would use 2n only, otherwise use the 2n-2 formula.

Netstudent

techster79 wrote:

They will specify if ip subnet zero is used or tell you about a classless routing protocol(hint hint) which you would use 2n only, otherwise use the 2n-2 formula.

From my experience, the 2 ^n was the default and (2 ^n)-2 was used if told to do so. The 801 exam will use standards from a current IOS. All else can fail and the difinitive decision is made by looking at the answer choices because both possibilites will not be in the answer choice. It's that simple.

dtlokee

mrj wrote:

I'm quite sure the CCNA (at least 801 version) is 100% NO ip-subnet zero, thus -2.

The difference is - in older IOS systems you COULD NOT use the subnet of, say, 192.168.0.x - It had to be at a minimum, 192.168.1.x.

In modern IOS subnet zero is common and you'll kick yourself in the head when you finish the CCNA because you'll be stuck in no-subnet zero land, which is a myth nowadays. But for the 801 there is NO subnet zero.

If your questions tell you specifically that ip-subnet zero is on or off, what I explained above is the difference.

Version 2.3 of the ICND (Since November 2006) it is 2**n, there is no longer a need to subtract subnet zero. I will say the question will typically imply what they want, subnet zero or not or the answer choice simply won't be there.

itdaddy

dumb question but

i thought all 1s was broadcast address and was being used
thus the -2

-1 is for all 0000s which ip subnet zero takes care of
-1 is for all 1111s which is still in affect

so why isnt it

2^n-1=s when ip subnet zero is in effect?

thanks