Internet is a worldwide network that conveys an expandable range of information resources and services which lead to bulk exchange huge traffic every day. Excessive popularity of internet creates some troubles in the networks. Flash crowd and Distributed Denial of Service (DDoS) attacks are the two major events among them. Botnets are the main drivers of cyber-attacks, such as Distributed Denial of Service (DDoS). Experienced botmasters attempt to disable detectors by mimicking the traffic patterns of flash crowds. During a flash event, a web server should aim to differentiate DDoS attacks from genuine flash crowds.