Subnet Help

new2net

Hey can someone please help me subnet this:

Something seems weird about the question...

A friend gave me this question to do, and I found it odd.

The company you work for has been given a Class B network number of 172.16.250.0. You have been asked to come up with a scheme to expand the network and you need to get about 40 subnets.

I need to figure out how many bits must be borrowed for the subnet and the resulting subnet mask.

Since the network that the company has been assigned is 172.16.250.0, I can only assume a mask of 255.255.255.0 has already been applied.

I know that 40 subnets will require 6 bits borrowed.

I know how to subnet, but I don't get why the question states we are given a Class B address when this address has already been subnetted, and the default Class B mask cannot apply since 250 is in the 3rd octet.
It seems as though they are asking us to subnet a subnetted address.

The question would have made more sense to me if we were given 172.16.0.0

new2net

Okay, so he checked the answer to the question.

It states that 6 bits must be borrowed for 40 subnets. I understand that completely.

The answer also states that the resulting subnet mask for the question should be 255.255.252.0.

This I don't get.

255.255.252.0 makes sense had we been given 172.16.0.0, but does not make sense when applied to 172.16.250.0

Unless I am totally missing something...

sandman748

I guess you could also subnet that given network as well resulting in a mask of 255.255.255.252 but that would only leave you with 2 useable hosts per subnet.

I think the question was just worded poorly. It probably expected you to realize that the class b classful address is 172.16.0.0 and go from there. I think your original thinking was correct.

new2net

Thanks sandman!

That's what I was thinking too.

It especially threw me off when he said that the answer to the question read 255.255.252.0 would be the correct mask. That goes against having 172.16.250.0 in the first place.

Didn't make sense to me.

schpenxel

You may also want to know that the 255.255.255.252 (2 real usable IP's) is the commonly used subnet in the world (or at least one of them). This is what is used for ISP's who hand out IP's to their subscribers as well as links that only end up needing 2 addresses.

So, I think it makes perfect sense for this subnet to be your answer. Since only the 4th ocetet of the network address (172.16.250.0) is 0, I would assume that that is the only one you can change. That would make the /30 subnet the correct answer in my mind

sandman748

It can also be argued that 172.16.250.0 / 22 would not even be a network. That address would be part of network 172.16.248.0.

It can also be argued that 172.16.0.0 - 172.31.255.255 is private addressing, so who would be assigning them a network block of 172.16.250.0?

I think he gets the idea behind the question. It's just worded poorly.

schpenxel

Agreed.