Information security is one of the most challenging problems facing network designers and operations managers. Along with viruses and worms, Denial of Service (DoS) attacks constitutes one of the major threats to the current Internet. Denial of Service attacks aims to crash a server or a network in order to paralyze its normal activity. Today, most organizations provide services over the internet hence an attack which targets their resources on the Internet. Denial of service is major class of security threat today. As attacker usually uses fake IP to hide their real location. One effective means to defend against such attacks is to locate the attack source and to filter out the attack traffic. To locate the attack source, this paper proposes an effective defense and IP trace back mechanisms. For implementing effective defense and trace back mechanisms against Denial of Service attacks such as SYN Flood and ICMP Flood we construct a simulation environment Using Network Simulator version 2.