A network intrusion detection system is mostly place at strategic points in a network, so that it can monitor the traffic travelling to or from different devices on that network. While choosing such a system, you should compare the main types of a network intrusion detection system. There are mainly two types of such system. One is signature based system and the other is anomaly based system. A signature based intrusion detection system is tuned towards a particular vulnerability, so it has less number of false positives (FP), whereas anomaly based system will search for attacks that are out of the norms, leading to higher rate of false positives. Therefore, you should choose a system as per your specific requirements.