subnet counting question

When presented with a subnetting question that, after laying out subnet and host numbers needed, asks to identify a certain subnet number (for instance, what is the broadcast address of the 4th subnet), should I count subnet 0 as number one in my counting, even is it is not considered valid by Cisco, or begin counting with the first VALID subnet according to Cisco? A recent study question I've seen seems to do it the first way.

Thanks for input.

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Comments

Webmaster

Indeed, the first way, as the first subnet 'can' be valid (depending on ip subnet-zero setting). Try a search in our CCNA forums for several topics on subnetting.

mehdi

for the exams,you shouldn't count the subnet zero as a valid subnet; even in practise you can counte it

mwgood

swtlrich wrote:

should I count subnet 0 as number one in my counting, even is it is not considered valid by Cisco, or begin counting with the first VALID subnet according to Cisco?

Go with what Cisco considers to be valid, unless otherwise specified - which means that you would normally use the -2 rule, unless you are told differently. The first subnet would then be the first VALID subnet.

If you have seen a question where it seems they are counting the zero subnet as the first subnet, could you post it? It'd be nice if we could have a look to verify that's how they are doing it.

It would seem to me to be a poorly written question if they actually had you start counting with the zero subnet instead of the first valid subnet - without telling you to do so.

tunerX

With the CCNA do not count subnet zero unless the question specifically states to do so.