Distributed Denial of Service (DDoS) attacks, one of the latest and most powerful threats that have appeared to the Internet can disrupt the availability of Internet services completely, by eating either computational or communication resources through sheer volume of packets sent from distributed locations in a coordinated manner or graceful degradation of network performance by sending attack traffic at low rate.An overview of DDoS problem, Vulnerabilities in Internet Architecture, Attack: Modus Operandi, different types of DDoS attacks, recent defense mechanisms and their effectiveness are presented. This provides better understanding of the problem, current solution space and future to defend against DDoSattacks.

Key words: Distributed Denial-of-Service (DDoS),Attack mechanisms, Defense mechanisms, Internet Security