Over at Linux.com is a look at five network monitoring tools that will really help you determine not only what is utilizing your network but also its overall health.  Bandwidthd is a tool for web hosts and LAN admins which tallies up incoming and outgoing bandwidth usage and can present it in a variety of time frames, from usage per day to usage per year.  Nethogs tracks usage per process and Darkstat tracks total usage but comes with an embedded HTTP server which makes it fairly self contained.  Speedometer is fairly self explanatory and to understand iperf you should read the write up here as it is a powerful tool for investigating the quality of your network.
“In this roundup of Linux network testing utilities we use Bandwidthd, Speedometer, Nethogs, Darkstat, and iperf to track bandwidth usage, speed, find network hogs, and test performance.”
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