Subnetting Class B

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.

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Comments

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.

bearfan

Thank you.

Default subnet=255.255.0.0

Had to borrow 9 bits to get 255.255.255.128

So 2 to the 9th power - 2 = 510.

255.255.255.128 will provide for both 260 subnets and 63 hosts.

I knew I learned that somewhere, but needed a refresher.

Thanks

calaverasgrandes

does anyone know where there are a bunch of example subnetting questions? (with an answer key or grading to evaluate yourself). I have just started "getting it" and want to work on some more to drive it home.

happy420golucky

calaverasgrandes wrote:

does anyone know where there are a bunch of example subnetting questions? (with an answer key or grading to evaluate yourself). I have just started "getting it" and want to work on some more to drive it home.

Maybe this site can help you. It doesn't evaluate you as you go, but it randomizes subnetting questions. Worked for me. Tootles
http://www.subnettingquestions.com/default_int.asp