Short for network intrusion detection system, NIDS is a system that attempts to detect hacking activities, denial of service attacks or port scans on a computer network or a computer itself. The NIDS monitors network traffic and helps to detect these malicious activities by identifying suspicious patterns in the incoming packets.
The NIDS can monitor incoming, outgoing, and local traffic. Inspecting outgoing or local traffic can yield valuable insight into malicious activities, just as inspecting incoming traffic can. Some attacks can originate and stay with the local network or be staged inside the network with an outside-the-network target. The NIDS also works with other systems, like a firewall, to help better protect against known attack sources (e.g. a suspected attacker IP address).
Also see: Security definitions