A smurf attack occurs when the broadcast address of a network is used to send packets to all hosts on that network. If network devices are not configured properly they allow such packets to be forwarded till they reach the target network. In such an attack, the attacker will send a large number of IP packets with spoofed IP address of one of the legitimate hosts on the target network. As a result all hosts on the network receiving the broadcast would respond by a unicast sent to the spoofed local IP address. This would cause a large number of packets sent to that hosts essentially resulting in a DoS attack on that host.