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Denial of Service (DoS) attacks constitutes one of the major threats and among the hardest security problems in today's Internet. Of particular concern are Distributed Denial of Service (DDoS) attacks, whose impact can be proportionally severe. With little or no advance warning, a DDoS attack can easily exhaust the computing and communication resources of its victim within a short period of time. Because of the seriousness of the problem many defense mechanisms have been proposed to combat these attacks. This paper aims to provide an understanding of the existing attack methods, tools and defense mechanisms, so that a better understanding of DDoS attacks can be achieved. The goal of the paper is to simulate an environment by extending NS2, setting attacking topology and traffic, which can be used to evaluate and compare the methods of DDoS attacks and tools. Based on the simulation and evaluation results, more efficient and effective algorithms, techniques and procedures to combat these attacks may be developed.