Subnetting Questions

new2net

Hello there... I need some help with subnetting please.

Well, I don't need help with subnetting, I mostly need help in understanding a question I am working on.

It is an example from the ICND1 book (it is on page 363 of the Second Edition if anyone wants to have a look). Here it is:

Your network can use Class B network 130.1.0.0. What subnet masks meet the requirement that you plan to allow at most 200 subnets, with at most 200 hosts per subnet?

The answer is 255.255.255.0

This is what confuses me.. The question says you plan to allow at most 200 subnets.

But using 255.255.255.0 actually allows us to use at most 256 different subnets. Also, it allows for 254 hosts and not at most 200 hosts per subnet

I understand how to subnet, but whenever I encounter a question that says "at most" I get confused... To me "at most" means we cannot go over that number.

So if a question (like the above one) says that we need at most 200 subnets, to me that means we are not allowed to have the ability to create more than 200 subnets with the mask. I thought the mask we create needs to enforce these restrictions.

When I first read the question, I thought a mask of 255.255.254.0 would be appropriate because it would allow for 128 subnets which is under 200. However it doesnt meet the other end of the question which requires at most 200 hosts/subnet.

Can anyone help me understand what I am doing wrong... Maybe I am thinking about it wrong...

I just dont get why the question asks for something like at most 200 subnets and at most 200 hosts per subnet, and then creates a mask that allows for more than 200 subnets and more than 200 hosts per subnet. I know there is no way to exactly restrict the mask to allow for exactly these amounts, so I don't get why the question is even worded that way.

To me, the works at most mean that we cannot go over this number. So if we need at most 200 subnets, then the mask we create should not allow us to exceed 200 subnets. Therefore we should use 255.255.254.0 because it will allow for 128 (because we are borrowing 7 bits). The second we borrow 8 bits from the 3rd octet we not have the ability to create 256 subnets, whereas the questions says at most 200.

Sorry, this is driving me crazy.

dynamik

This was a bad question of when to use "at most" since it used it for both portions and there weren't any flexible bits. This is just my advice btw, but when you see one section asking for "at most", I would choose the least number of bits required to meet that number. You're obviously not going to meet that requirement if you choose one bit lower (like 7 in this example).

I think a better question would be with the same class B network and require at most 200 hosts and at least 100 subnets. See how that works? You'll have one extra bit you don't know what to do with. Since you know you'll never need more than 200 hosts, you should only use 8 bits for that one. However, the question seems to leave open the possibility that their might be more subnets. Maybe that number will go to 130, so you should also use 8 bits there instead of just using the 7 necessary to meet the requirement of 100.

It's just obscure wording, and I wouldn't worry too much about it. I hope that helped, but I might have confused you more