Network Security Monitoring can tell you what systems live on the network and what they are doing. It is low-cost compared to the variety of alternatives (manual, physical asset control; network access control; scanning; etc.) Once a sensor is deployed in the proper place you can perform self-reliant (i.e., without the interference of other groups) NSM, on a persistent and consistent basis, is working right now. Richard Bejtlich is teaching new classes in Europe and Las Vegas in 2009.”]
Source: https://taosecurity.blogspot.com/2009/03/network-security-monitoring-lives.html