# 2^n-2 subnet bits and networks question

Member Posts: 69 ■■□□□□□□□□
I was going over some 'challenge problems' in my exam prep book and I got to this one that I can't figure out.

network bits=
host bits=
subnet bits=
subnets =

for network bits I have 24 since it's a class c address, host bits I have 8 because there was no mention of CIDR going on. for subnet bits I had 0, because no bits were left over from the network/host bits. now for the last one, using the formula the book told me i should use, where n was the number of subnet bits I had 2^0-2 so I would get -1. that doesn't seem right.

so, I'm kinda stuck on this one. It seems like it should be easy but I can't figure it out for whatever reason. Am I getting the subnet bits and total subnets completely wrong on this one?

here is a completed version of this problem for reference

222.110.8.61/28
network bits 24
host bits 4
subnet bits 4
subnets 2^4-2=14
hosts 2^4-2=14

simple, yet I can't figure out the first problem still. I'm missing something

• Member Posts: 2,378 ■■■■□□□□□□
I'm not sure what book you're using but the current version of the courseware will tell you to use 2**s to calculate the number of subnets because "ip subnet-zero" is on by default.

In the example you have here it is a class "c" with a /28 subnet mask. The default mask for class c is a /24 (and this is the same as the number of network bits). This means there are 24 network bits, 4 subnet bits (28-24) and 4 host bits (32-2. The numer of subnets would be 2**4 which equals 16 (they subtracted 2 and that is how they got 14) and the number of hosts er subnet would be 2**4 - 2 which equals 14 hosts per subnet.
The only easy day was yesterday!
• Member Posts: 69 ■■□□□□□□□□
thanks for the reply, the problem you went through was the one I was able to complete already, but that helped because you confirmed that I at least did that one correctly. The one I can't figure out is the first one I listed, because it seems the subnet bits are zero because it's a conventional Classful address, no /insert bit here.

Is that a trick question? I'm starting to think it is, because having 0 for subnet bits leaves you with -1 total networks...I don't think that's correct. maybe there is another formula I'm supposed to know and use in this case, but I don't know what it is.

BTW the book I am using is Exam Prep Second Edition CCNA 640-802 Page 154. I'm thinking there is a reasonable chance someone else here has this book too.
• Member Posts: 69 ■■□□□□□□□□
I'm not sure if that question was just a mistake, because I still can't figure out exactly what the author was going for there.

I've found several mistakes in the book, the exam prep by Jeremy Cioara, and I'm only on page 161 out of 900 something.

For instance, I was just learning about IPv6 and he states that in binary link-local addresses are always 1111 1110 0100 0000. That would give them a hex of FE40. However, as I'm sure most of you probably know they always start with FE80, at least to my limited understanding.

If this is really a mistake on his part I can only wonder what other nuggets of misinformation are hidden away in my subconscious lol. I tend to remember things the way I see them the first time...

There was also a huge mistake in the section regarding how to learn dotted-decimal. Hard to learn if the instruction is wrong to begin with.
• Member Posts: 56 ■■□□□□□□□□
BTW the book I am using is Exam Prep Second Edition CCNA 640-802 Page 154. I'm thinking there is a reasonable chance someone else here has this book too.

I'm using the same study book but haven't gotten that far. Hope to be able to check this out over the coming weekend.
Regg'

"Life without knowledge is death in disguise."