Number of DoS attacks used to breach target networks are increasing every day. Network infrastructure is the backbone of a company. Once this network is down, organization has to face various losses in terms of finance, reputation, confidentiality, integrity and availability. DoS attacks are the most widespread attacks executed on organization networks. Its main aim is to prevent trusted users from accessing legitimate services. This book suggests some upgraded defense mechanisms against these attacks which include ARP spoofing attack in wireless LAN and DNS poisoning attack in wired LAN.