NIDS

An IDS is a system to track any changes not planned to a system. It is often used on sensitive machines where any unauthorized access is purely prohibited but can also act as a fool-proof system, more like a monitoring system. It works by checksumming or understanding the format of each file, and scrutinizing any suspect change to files.

A NIDS is a Network Intrusion Detection System, a system to detect intrusions from the network and take counter-measures ranging from (temporarily) blacklisting to logging through responding with specially crafted IP packets to slow down or kill the attacking host. It extends the principle of IDS to the network.