Any secure system must be protected from attacks. A good defense is made up of two types of action. The first is a passive defense consisting of knowledge, effective procedures, and equipment properly initialized and maintained. These things must be done before any system is brought on-line. The second is a plan of what to do when your system is attacked.
Intrusion detection is a blossoming science, involving detection research and reaction research.
Intrusion systems have largely been manual processes. The intrusion detection body of knowledge is growing rapidly. There are many new areas being explored and many older standard methods being upgraded. From this position, network traffic is monitored before it is allowed to enter an intranet system. This labor-intensive method prevents access to a system's intranet infrastructure.