The advances in information technologies in the internet are increasing the possibility of attacks exponentially. Denial of Service (DoS) and Distributed Denial of Service (DDoS) attacks are increasing rapidly across the internet world. Denial of Service (DoS) is an attack on availability of a service. The attack aims at denying of an approved service to a legitimate user. When a group of attackers perform DoS attack on a common target, the attack is known as DDoS. The most common method of performing a DoS or a DDoS attacks is to flood the target or network with unwanted traffic, causing interruptions to the communication of legitimate users. The attacks are evolving in a way, the frequency and the severity, sophistication of DDoS attacks are increasing very fast. Existing methods of DoS/DDoS attack, defence mechanisms are outdated and even the latest surveys do not re?ect the signi?cant developments in this area in recent years. This paper will explain, in simple terms, the various types of DDoS attacks and the technologies that are used to prevent and mitigate the attacks. This comprehensive study of attacks and their defence mechanisms would provide the researchers with a better understanding of the problem and the possible solutions.

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