A network-based IDS system monitors the traffic on its network segment as a data source. This is generally accomplished by placing the network interface card in promiscuous mode to capture all network traffic that crosses its network segment.
Network traffic on other segments, and traffic on other means of communication (like phone lines) can’t be monitored. Network-based IDS involves looking at the packets on the network as they pass by some sensor.
The sensor can only see the packets that happen to be carried on the network segment it’s attached to. Packets are considered to be of interest if they match a signature.Network-based intrusion detection passively monitors network activity for indications of attacks. Network monitoring offers several advantages over traditional host-based intrusion detection systems.
Because many intrusions occur over networks at some point, and because networks are increasingly becoming the targets of attack, these techniques are an excellent method of detecting many attacks which may be missed by host-based intrusion detection mechanisms.