My ISP and Bandwidth Usage - I Think I'm Getting Screwed

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So this month I've been getting hassled by my ISP for going over my allowed bandwidth. I'm allowed 60GB to upload/download a month and apparently I've used 100GB. I know that it's impossible that I've used this much since aside from standard web browsing and pokering I've only downloaded/uploaded about 20GB of data. And as a reference, they are reporting that last month I used a combined bandwidth of 25GB.

After having my service disconnected as a warning, I was left with 1GB to use for the rest of this month. To make sure I stayed under this limit and didn't get disconnected again, I've been running a bandwidth monitor on my two computers. This monitor records everything uploaded/downloaded from the internet as well as everything that goes between my two computers on my local network.

Using the data from my monitor along with the bandwidth usage data that my ISP has available for me to view online, I've found that they are not only counting what goes between my modem and the internet, but they are also counting the data that is being transferred between my two computers through the router on my home network.

This would definitely explain the excessive usage since this month I've been using one computer to watch tv shows and movies that are on the second computer. This would account for about 40-45GB of transferred data.

Someone please tell me that this is completely wrong and that I can call my ISP and bitch at them. I'm not overly computer literate, but it makes no sense that they could count data going through my home network as bandwidth usage when I could be doing it without being connected to the internet at all.

Thanks to anyone who helps me out with this.