IPAcco is a free software package that helps a network admin 
to collect, visualize and analyze IP accounting data from the 
Cisco routers. 
Cisco routers themselves are capable of collecting IP accounting 
information . i.e. an unordered set of IP source-destination 
pairs along with a byte and packet counters corresponding to all 
network traffic flows that passed through the router's interfaces. 
These data can be a useful source for various analysis procedures 
and billing systems but by itself, in their raw form they are 
rather difficult to read and understand. In addition, a router 
cannot keep a lot of data . its memory is needed for purposes 
other than remembering what traffic, from what sources and where 
it forwarded two month ago.

WWW: http://ipacco.sourceforge.net/