Making a passive network tap can be an easy and inexpensive undertaking as shown in this Instructable. Passive monitoring or port mirroring is needed because most networks use switches which isolate the network traffic and this does not allow for the entire network to be monitored. This example uses a single tap, using multiple taps will provide access to the full-duplex data separately. By using two taps you are able to monitor inbound data that is passed through one tap, and outbound data that is passed through the other tap. Separate taps are desired because most sniffer software handles half-duplex traffic only and requires two network cards for full-duplex.
