How to monitor network traffic on a router from a Mac? A company has a High Speed internet connection (30 MB/s) which is enough for 10-15 computers. But for a week, there is almost no bandwidth "left", just checking email takes a long time or fails. Getting a website on screen is almost impossible. I checked with speedtest.net and now the speed is 3mb/s to 6 mb/s if not zero in the test time. 

So the question is how I can monitor the in and out from the modem/router and analyze what IP take all the "juice" to take that bummer down in flame (or just find the program that make the internet choke). I have a Mac running 10.6.8 (the latest Snow Leopard release) and would like to better manage the network infrastructure and gain insight into how and when it becomes overloaded or performs poorly.

So in short, I need to track which IP or MAC address makes the most requests... And BTW, when there is no person at any computer (over the w-e) the line is fully used (as said by the internet provider (videotron) company) which is not supposed to happen!

NetUse monitor has been suggested by many, but I am still wondering about the features I need. Here is my problem and tell me how your software or other can help me.
Put a switch which can have ports configured in "span" mode so you can sniff all the traffic, (an old hub will also do the same job) between the router and the base station.

Plug in your laptop, install wireshark and sniff all the traffic.  Then analyse.

The other tool I use is MRTG, but you will need to set up SNMP on the Airport or the "spanned" switch, and monitor for a few days.

You can definitely use SNMP with an Airport extreme as documented here.