An intrusion detection system(IDS) is a software system or hardware device that inspects all inbound and outbound network traffic for suspicious patterns that may indicate  a network or system security breach.The IDS checks traffic for signatures that match known intrusion patterns and signals an alarm when a match ks found.Depending on the traffic to be monitored,the IDS is placed outside/inside the firewall to monitor suspicious traffic originating from outside /inside the network.