All computer security breaches cannot be prevented using access control Mechanism and information flow control technique. These breaches may be due to System software flaws, improper system administration procedures or failure of the Authentication module. Intrusion detection can play an important role in this type of Breaches. This book concentrates on intrusion detection as a second line of Defense after the access control mechanism in the system. This book provides an Solution to the given problem: Can we usefully and effectively detect site security Violations happening in the system when the access controls mechanisms fail? A More descriptive form statement is: An intruder may get access to the system by overcoming the access control mechanisms under certain circumstances. Therefore, important events occurring in the network that are of interest to the security are logged in the system. These logs can be analyzed to infer any site security violations that could have happened in the network with architecture of De- centralized Distribution Intrusion Detection system.