I have been using this simple TCP Test Tool on Windows. It is a very basic GUI for listening ports and making TCP requests, yet it was very handy. However I could not find anything similar which works on Mac OS X.
Does anybody have any suggestions?
It's great, powerful, but the free means you will spend some time getting it set up and running since it's not like buying a desk but more like directions and only the hard to obtain parts needed to make yourself a desk.
You can download all the source for these and compile the programs. For someone familiar with command line and building open source unix tools it might be 30 to 90 minutes of effort to figure out any build errors and correct them.
I don't see an up-to-date package that lets you download and run Zenmap and it runs under X11 so if you are new to X11 - do read up a bit here or on the Apple discussion boards - it's just a little different than a normal mac program so you might also spend some time learning X11 before you will be running the tool.
I don't mean to make any of this seem overly hard. A motivated but absolute beginner can get this all done in a day with luck - just that it's not something you will have running in 5 minutes. Your curiosity to debug networking might make this a great journey in learning how the software really works by looking at the code and seeing the tool actually run.
You might also find Wireshark useful. I found a good video on Youtube to getting it setup.
All the steps in the video may no longer be necessary as the changelog for version 1.6.0 says the following: