Security has become the need of the hour as no organization is immune to data breach. A recent survey reports that 16% of attack on network is due to denial of service attack, which prevents legitimate users from accessing services or information. This paper focuses on ping flood attack, ping of death, SYN flooding and defense mechanism to defend the nodes being attacked in real time. Ping flood attack is successful when an attacker overwhelms the server by sending bogus redirect requests than the server can process. It is a kind of denial of service attack where online banking, email, and commercial websites are often targeted. Ping of death is when the size of the ping packet exceeds the normal packet size. SYN flooding is a situation when the victim is unable to accept and serve new requests due to half open connections. Quality of service (QoS) parameters such as data dropped due to buffer overflow, delay, load, retransmission count, and TCP segment delay are analyzed for different scenarios and graphs are plotted using OPNET simulator. We show that there are possible solutions to recover network from losses incurred due to attack by employing firewall to block all traffic coming from an attacker by identifying the IP, varying segment size, etc.Keywords: Ping flood, ping of death, SYN flooding, OPNET simulator, firewall
