A ping flood is a simple Denial of Service (DoS) attack where the attacker sends a massive number of ICMP Echo Request (ping) packets to the victim in hope of overwhelming the victim and consuming both outgoing and incoming bandwidth on the victim's network. An effective attack could render the victim's network useless.
This type of attack only succeeds if the attacker has more available bandwidth than the victim (like a T3 versus and DSL line) which is very unlikely. To get around this, the attacker normally use an army of computers (a network of computers that attacker has control called zombies or bots) numbered in the hundreds or thousands to attack the victim with their combined bandwidth power.
Defense Your Network
To reduce the effects of ping flood attack, one can use a firewall to inspect the traffic and filter ICMP Echo Request packets. However, doing this can also have its side effects such that it prevents legistimate users from pinging their hosts. For this reason, we can use firewall to authorize ICMP Echo Request packets from legistimate source only.