Subnetting Class B

bearfan

I'd like to understand this better:

A practice question asks something similar to this:

You have been assigned a network address of 172.31.0.0. You need 260 networks and 63 hosts. What's the subnet mask?

I can compute for 63 hosts - 255.255.255.128. However, this would provide only 2 subnets for a class C address. This is class B. How do I know this would provide for 260 networks? I know it would provide more than 2 subnets since this is a class B address. Do you just pay attention to the number of hosts? I get the question right that way, but remember on the test I could get asked questions from different angles.

Thanks everyone for all the help.

Netstudent

How do I know this would provide for 260 networks?

You have to start counting the bits after the classful boundries. You knew it had to be class B right?
So the default subnet mask is 255.255.0.0. Anything after that, start counting.

255.255.255.128 gives you 9 subnet bits. Plug that in the your formula.