A few years ago, when we spoke of network intrusion security systems, we spoke of IDS (Intrusion Detection System) appliances. Recently, as the emphasis has shifted from detection to prevention, IDS has become IPS (Intrusion Prevention Systems). The compelling force behind this change is the same one that has thrust an open source software company named SourceFire to the front of the Network Intrusion Prevention System Appliances market sector; that is, a fast changing threat environment.
In an article for Military Information Technology, Deputy Undersecretary of Defense Sue Payton writes that

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