Here is something I've been meaning to write for some time. IP subnetting is one of those topics that is useful for any IT person, especially network engineers or anyone who deals with Operating Systems.
I've helped many friends on this topic over the years, but I finally turned this in to an article. To make the topic as easy as possible, I put in a lot of graphical illustrations to simplify the concept. You can get the finished article in HTML format, or download the PDF version.