In order for Tcpdump to work, it must be able to put your network card into what is called promiscuous mode. This means that the network card will intercept all traffic on the Ethernet wire, not just that addressed to it. Each operating system processes traffic from the Ethernet card in a different fashion. To provide a common reference for programmers, a library called pcap was created. On UNIX this is known as libpcap and on Windows as WinPcap. These low-level drivers can modify the way the card would normally handle traffic. They must be installed before you can install Tcpdump.