Intrusion detection systems fall into one of three categories: Host Based Intrusion Detection Systems (HIDS), Network Based Intrusion Detection Systems (NIDS), and hybrids of the two.
What is an Intrusion Detection System? An intrusion detection system (IDS) is a device or software application that monitors a network for malicious activity or policy violations.
Any malicious activity or violation is typically reported or collected centrally using a security information and event management system.
NIDS are placed next to the firewall on the network perimeter and analyze the traffic as it passes by for the protocols, source, destination, content, traffic already seen and etc.