Intrusion detection systems (IDSs) provide an additional level of security for your network. It is worth noting that unlike firewalls and VPNs, which attempt to prevent attacks, IDSs provide security by arming you with critical information about attacks. Thus, an IDS can satisfy your demand for extra security by notifying you of suspected attacks (and, sometimes, of perfectly normal events, through "false positives").
IDSs, in general, do not actively block attacks or prevent exploits from succeeding; however, the newest outgrowth from network IDSs—the intrusion prevention systems (an unfortunate marketing term)—strive to play a more active role and to block attacks as they happen.
Defining an IDS is harder than it sounds. Early on, IDSs were viewed as burglar alarms that told you when you were being hacked. However, the modern IDS world is much more complex, and few would agree that IDSs (at least, network IDSs) are at the same level of reliability as conventional burglar alarms. If improper analogies are to be employed, network IDSs are more akin to security cameras than to alarms—a competent human being should watch them and respond to incoming threats.
Indeed, IDSs sometimes might only tell you that your network has just been trashed. The important thing to realize is that few hacked networks get this luxury in the absence of an IDS. As we have seen, a network might become a haven for hackers for years without the owners knowing about it. ...