Networking is one of those topics that generates a vast number of questions. How fast is my Internet connection? What IP addresses are in use on my network? How can I improve the performance of my wireless network by adjusting the position and orientation of my AirPort base station? Is my ISP on a DNS blacklist that's preventing my mail from getting through? Are there any services running on my Mac (and creating a security vulnerability) that I didn't realize were active? Is my server crashing regularly in the middle of the night?
There are a variety of tools that can answer some of these questions, particularly if you're extremely comfortable at the Unix command line, but if you're a Mac user who prefers a graphical interface, a better option is Sustainable Softworks' IPNetMonitorX. Run by networking guru Peter Sichel, Sustainable Softworks has built up a reputation for creating networking tools that look deep into the guts of the Mac OS to answer all the above questions and many more.