The freeware network protocol analyzer Ethereal does what you're probably
looking to do.  Just capture a sample trace of network traffic going into
your router (insert a hub in a link you want to analyze, and hook up a
laptop with ethereal installed on it.  Make sure you use a hub, not a
switch), hit statistics, endpoints, and it'll tell you how much traffic is
coming from/going to each endpoint on the network.  It'll do it at either
layer 2 (mac addresses), or layer 3/4 (ip addresses/tcp ports).  There's
enough info there to tell you what sort of traffic, too.  Once you know
who, you'll want to know how, and why. 


School Information Technology Discussion <[log in to unmask]> writes:
>Grr,  My network / internet keeps grinding to a halt at certain times of
>the day. 
>
> I would like to find a way to see how much network resources each
>computer is using, so i can find out why I have a bottle neck (virus, new
>download program, video streaming, a specific class,  ...).   I have a
>cisco pix firewall, Cisco routers, and n2h2 content filter on its own
>server which all internet traffic hits.
>
>If anyone has any recommendation I would greatly appreciate it.
>
>Bjorn Behrendt
>Proctor School District
>Tech Director
>[log in to unmask]
>Phs: (802) 459-3353 x2113
>Pes: (802) 459-2225 x2018