Our office LAN is getting painfully slow. It's likely that something is hogging the bandwidth.

If I were to put a network hub between the ADSL router and the main office switch, presumably I could run something on a PC also connected to that switch which would point me in the direction of the problem?

I'm looking for something fairly simple, that would tell me (eg) which IP address was generating most of the traffic, and perhaps on which ports. Wireshark would probably do what I need but it's not (from what I remember) particularly simple to get simple results from.

Or, is there a better option?

"iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question "why is our ADSL link so slow?"