Subnet question. Am i missing something?

albanga

Hi All,

I have a subnet question that appears easy to me but I got it wrong. I would love some feedback as to why?

You have a class B network with a 255.255.255.0 mask. Which of the statements are true of this network? (Choose two)

A - There are 254 Useable subnets
B - There are 256 usable hosts per subnet
C - There are 24 usable hosts per subnet
D - There are 254 usable hosts per subnet

The answer I was given was A and D. Whilst I agree D is correct, shouldnt it be 256 useable subnets?

My Math is;
255.255.255.0 translates to /24. So because it is a class B network it would be 24bits - 16bits, leaving you with 8 bits. 8^2 = 256. Seeing as though it is the subnet there is not a need to minus 2 like the host portion. Am i correct, or am i missing something?

Please tell me the question has got it wrong

mikej412

albanga wrote: »

Please tell me the question has got it wrong

What's the source of the question? It sounds like it may be an old source not allowing use of subnet zero.

In the future post the source of the question (and chapter/page) so people with the book can look it up for themselves and check the context.

Forsaken_GA

back when I went through the network academy for ccna, we were taught that the first subnet and the last subnet were unusable, because they contained the network identifier and the broadcast for the entire classful network, and routers didn't know how to handle that.

these days it's an obsolete concept, routers are perfectly capable of using all subnets. However, it's possible that you may encounter legacy deployments where the old rules still apply.

Assuming this is from a book, check the section on subnetting to see if that's what they teach. If so, then answer A is correct. Just be aware that for years (and I don't know if it's still the case, as I haven't looked at current CCNA material) Cisco taught to not use the all zero or all ones subnet as a matter of gospel, after which they retconned your reality at higher certification levels

chmorin

Dealing with cisco, if it is not directly stated in the question, assume you cannot use ip subnet zero. This would make answer correct by cisco standards.

Real world and exam do differ.