Opposed to active monitoring that involves the injection of test packets in the network and the consequent measurement of their qualitative characteristics, the approach for communications network monitoring referred to as passive concerns the inspection of the actual network traffic using special software and/or hardware equipment. Enabling factors for the recent surge of passive network monitoring have been the availability of high-performance capturing devices, such as network probing cards, and storage systems of large capacity, as well as the development of intelligent methods for traffic analysis and knowledge inference.