If it's unencrypted (it is): Use a network hub + ethernet (not a switch) OR: use wireless OR: Make your router/switch replay the packets to a port of your

If it's unencrypted (it is): Use a network hub + ethernet (not a switch) OR: use wireless OR: Make your router/switch replay the packets to a port of your choosing. Then most people use wireshark. But you can use tcpdump/pcap or whatever else you want.