Here is a free subnet calulator that is perfect for the job…
If you want a simple explanation on how to calculate subnets, this tip by Mimi Shaw is a great resource. It also has a quick quiz at the bottom to test your subnetting skills. The tip is called: <a href=”http://searchnetworking.techtarget.com/tip/0,289483,sid7_gci1284276,00.html”>How to subnet: Subnetting calculations and shortcuts</a>
I hope that helps!