Abstract
A denial of service attacks are usually employed to interrupt the system's activity by getting a large number of fake requests, which leads to slowing down and stopping information system or network operations. This paper outlines the basic principles of DDoS attacks, explains how DDoS attack works. The paper presents an experimental using OPNET simulation tool in which traffic from three different applications VoIP, FTP, and HTTP is used to make the practical model with a firewall shield to model and simulate DDoS over Internet. However, three scenarios have been developed to illustrate different aspects of the DDoS over networks. The results clearly demonstrate how firewalls can be configured to mitigate DDoS attacks