[CentOS] Network accounting

Hello,

we have been using squid cache under CentOS 6.
There is basic accounting for each host/client but we need
to account for each subnet. Can you recommend any software
which easily let us do subnets network accounting ?
E.g.:
192.168.1.x	X Gb
192.168.2.x	Y Gb
192.168.3.x	Z Gb
...


Regards
Przemek