Subnetting Question (Subnetting Class A networks Video)

Benjamin Frame

Hello All,

There was a question outlined in the CCNA subnetting video (Subnetting Class A) and I was wanting to confirm the correct answer. ( I understand this is neither a class A IP address nor a class A subnet. This question was just outlined in the Class A subnetting video)

Anyhow...

The question is as follows

You've been assigned 172.31.255.128/26, how many subnets can you create?

a. 4 networks
b. 2 networks
c. 256 networks
d. >400 networks

I am just refreshing but I feel this is what is called a classless IP address and given the subnet mask, there would be 4 total available subnets we could create.
172.31.255.0
172.31.255.64
172.31.255.128 is the subnet ID. ( Incrementing by 64 )
172.31.255.192

Thank you,

Ronnie Wong

@benjamin-frame,

Not exactly,

So the number subnets you can create start at the classful boundary. In this case
172.31.255.128/26 is in the Class B boundary. So that for us would mean that the default subnet mask would be /16 or 255.255.0.0.

So what you would do in this instance is you would begin is to take that subnet /26 and subtract /16 to give you the number of subnets you can create for this network. This would equal you have = 10 bits of subnets. This is 2^10 which is 1024. so in this answer d.

What you did is treat the assigned subnet as a Class C. address. But the reality now that I read it is a poorly worded question. So my apologies on that front.

Benjamin Frame

So I understand your response as to, borrowing X amount of bits that equates to # of subnets but can you explain how it should increment.

Also, there are classless IP addresses where a traditionally Class B IP is used with a class C subnet for instance. So if it's not defined in a question, is there more than one possible right answer?

On a side note. I appreciate your humbleness in your videos. So not trying to give you a hard time. I am just trying to understand is all.

Appreciate your help,

Thanks,

Ronnie Wong

@benjamin-frame said in Subnetting Question (Subnetting Class A networks Video):

So I understand your response as to, borrowing X amount of bits that equates to # of subnets but can you explain how it should increment.

If you're speaking about subnetting, You start with the Classful boundary, unless you're subnetting an already subnetted network.

Also, there are classless IP addresses where a traditionally Class B IP is used with a class C subnet for instance. So if it's not defined in a question, is there more than one possible right answer?

This is subnetting a subnetted network, which you can do. As I stated in my previous answer. This question was a bit vague in what I was asking you to do. But the reality here is that 172.31.255.128 is part of a larger network already that began at 172.31.0.0/16. Classless or classful this is where you begin. On something like this, for an exam. It will not be as vague as I am here.

On a side note. I appreciate your humbleness in your videos. So not trying to give you a hard time. I am just trying to understand is all.

No problem.

Appreciate your help,

Thanks,

Benjamin Frame

Ok, perfect. Thanks Ronnie. Your help is appreciated,