howto monitor net bandwidth of other users in the network

[wanas]

I want to monitor the bandwidth of other users in the network to know who eats the net speed.
- I am using ubuntu 9.04 beta.
- I have a Tp-Link TD-8840 router. (I have enabled snmp)
- The other users in the network is using windows xp.
- I have installed cacti 0.8.7b via the synaptic manager on ubuntu.
- I am not a professional so teach me with apples

[gandalf]

SNMP odes not report traffic related to specific IP adresses. Cacti uses SNMP by default and as such suffers from this.
There are several plugins that mjay help you, e.g. flowviever.
Another option would be to simply measure the traffic of each of the other hosts.
The last bet would be a different tool like ntop
Reinhard