Now-a-days, as information systems are more open to the Internet, the importance of secure networks is tremendously increased. Distributed Denial-of-Service (DDoS) attacks are a critical threat to the Internet. However, the memory less feature of the Internet routing mechanisms makes it extremely hard to trace back to the source of these attacks. DDoS attacks are becoming more powerful and more complex, and will pose an increasingly serious challenge. Of particular concern is the increased threat to the financial services industry, which has been targeted by hackers at unprecedented levels. New intelligent Intrusion Detection Systems (IDSs) which are based on sophisticated algorithms rather than current signature-base detections are in demand.