If you work on a medium to a large size network, then you need to understand IP subnetting? IP subnetting lets you breakup a large network into smaller sub-networks. Personally I find this subject one of the more difficult parts of networking to understand, mostly because you don't use this knowledge very often.
If you want to understand this topic here is an an excerpt from a TechRepublic article on the subject: "IP subnetting is a fundamental subject that's critical for any IP network engineer to understand, yet students have traditionally had a difficult time grasping it. Over the years, I've watched students needlessly struggle through school and in practice when dealing with subnetting because it was never explained to them in an easy-to-understand way. I've helped countless individuals learn what subnetting is all about using my own graphical approach and calculator shortcuts, and I've put all that experience into this article."