A network-based intrusion detection system (NIDS) is a virtual or physical device that is applied to the network as a whole and not an individual device. It is very similar to HIDS but the main difference is that the monitoring is based solely on network traffic. Instead of installing an agent on a machine, the NIDS uses using either a network tap, span port, or hub collects packets that traverse a given network. NIDS will use the captured data from the network so that the IDS system can process and flag any suspicious traffic. Remember that a NIDS is an IPS and thus doesn't act it just monitors. A great example of a NIDS is Snort, and we will be using Snort later in this chapter.
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