Yes, this is the correct forum for Web Reporter questions as well as Web Gateway questions. I understand what you're trying to accomplish, however I don't believe it can currently be done with a single report. However, you could create a query which displays the bytes used by an individual IP address, then displays the sum of all bytes used. You could run this report multiple times, using an IP filter that you've created for each 24-bit subnet. That would give you a report on the total bandwidth consumed for each subnet, but you'd have to compare the results manually.
Hopefully this helps in the short term. However, if you'd like to see a feature like this added to future versions I encourage you to contact your sales representative. He/she can make sure this bubbles up through the proper channels to have it considered as a feature for future releases.
I suspected this might be the case. At the moment we are putting in separate reports with subnet range filters, but with over 400 different locations to report on its not easy to get a decent site level overview. I'll put a feature request in and hopefully my wish will be granted