traffic monitoring

[Carrot Cruncher]

does anyone know if i can report on the network traffic passing through a 7800 based on either source or destination ip address? i currently have cacti (http://www.cacti.net/) producing reports from the snmp data for the overall incoming and outgoing traffic but i can't work out how to produce a report based on ip addresses. i've done an snmpwalk of the router and it returns lots of interesting looking data but i don't know snmp well enough to know if i can achieve what i want. thanks.