For anyone still struggling with subnetting and all those fiddly binary numbers when working with IPv4 addresses, help is at hand in the form of an excellent article in a back issue of The Internet Protocol Journal.
This article takes you through the basics of IP addressing and provides an easy strategy for working with those numbers for calculating an appropriate subnetting strategy. It has a heading “The Hardest Subnetting Problem” which looks much like an examination past question (only harder!), and a fully worked example. It also speaks briefly about working with IPv6 addresses in the summary.
Well worth a read.