If your network is slow or experiencing other problems, one tool to help figure out what’s wrong is a network sniffer (also called packet analyzer).
Some free multi-platform network sniffers include Ettercap, ngrep, tcpdump, and Wireshark.
To get started with network sniffers, Wireshark is a good place to start, as it has fairly extensive documentation and support, and features both GUI and command-line style interfaces.
If you want to use Wireshark, don’t go to www.wireshark.com. The address is www.wireshark.org.
In addition, you’ll want to check out the following informational sites:
Wireshark user guide: www.wireshark.org/docs/wsug_html/
Wireshark training: www.wiresharktraining.com/