The original tool was really intended to be a command line utility, but I was prompted to port it to web so I did. It's all written in python 3/django, and I had a hell of a time getting django set up on my server. I still need to put it on toolserv/whatever we use these days.
I don't think anyone has any sort of IPv6 working with that tool. Correct me if I'm wrong but I always assumed there was another issue with the implementation. I'll review in the AM, although I'll have to learn some js in the process.
It was not arbitrary. At the time rSVN47239, 96 was the max limit for IPv6. The patch above changes it to use the current value of $wgCheckUserCIDRLimit['IPv6']. I guess we could later change it to use the value directly from config.
I have tested it and it does work. After the change, it would now work for ranges smaller than /32.