As 100volts said, there are many ways to measure traffic from a specific computer or on a LAN.
The issues that have not been solved for the
HN9000 that hnFAPmon solves for all the legacy modems are 1) the issue of monitoring on a rolling 24 hour basis, and not a calendar 24 hrs, and 2) refilling alloted bandwidth concurrent with depleting that same allotment.
As you contemplate
additional equipment or software, you need to keep those two issues in mind and whether they are being addressed.
I haven't seen a decent solution yet. There is a shareware opportunity laying out there, it just hasn't been picked up yet by a software developer.