Subnetting Question!

viper75

I have been doing a few practice test for the CCNA and one of the questions I got goes as follows:

Your organization is using the 198.133.219.0/24 address space for it's Internet presence systems. The IP address plan provides for the 4 subnets within the given address space. What network mask are these host in your organization using?

A)255.255.255.224

B)255.255.255.8

C)255.255.255.4

D)255.255.255.192

E)255.255.255.0

I thought I picked the correct answer, but I got it wrong. Can you(whoever answers this post) tell me why you picked the answer you did. I am a bit confused by this.

Maybe I'm not understanding the question right.

mwgood

B & C are invalid subnets - so those are ruled out.

They are providing for only 4 subnets -

2 bits on = 2^2-2= 2 subnets
3 bits on = 2^3-2= 6 subnets

and...

3 bits on = .224 so,

Answer is A, 255.255.255.224.

dissolved

Here's where I'm confused. If they say they are using the 198.133.219.0/24 address space, then why isnt the answer E) 255.255.255.0

Doesnt the "/24" denote a 24 bit subnet mask? Which would be answer "E"? Help

viper75

Ok...maybe there was a mistake on the practice exam...because I had picked Answer A) 255.255.255.224, too but they marked it wrong. They said the correct answer was D) 255.255.255.192. That's why I got confused!!!

I guess there is a mistake on the practice exam question\answer.

mwgood wrote:

B & C are invalid subnets - so those are ruled out.

They are providing for only 4 subnets -

2 bits on = 2^2-2= 2 subnets
3 bits on = 2^3-2= 6 subnets

and...

3 bits on = .224 so,

Answer is A, 255.255.255.224.

mwgood

Right. Only way the answer is D is if they are not using the -2 rule, and allowing subnet zero and the all 1's subnet, then it would be...

2^2=4 subnets. So the answer would then be 255.255.255.192 - but... the rules that Cisco uses for the CCNA (unless otherwise specified) is the -2 rule for the number of subnets.

dissolved

mwgood wrote:

Right. Only way the answer is D is if they are not using the -2 rule, and allowing subnet zero and the all 1's subnet, then it would be...

2^2=4 subnets. So the answer would then be 255.255.255.192 - but... the rules that Cisco uses for the CCNA (unless otherwise specified) is the -2 rule for the number of subnets.

mwgood, can you take a look at my question when you get a chance? I'm confused. It's up top a little in this post
thanks man

mwgood

The reason why it's not 255.255.255.0 is because they are dividing the /24 into 4 subnets.

dissolved

Thanks man

mnaz

the /24 is your network portion of the address meaning that you only have the last 8 bits of the address to subnet.