Denial of service attack, is an attempt through some means to make a computer or network resource unavailable.

Some systems are susceptible to a simple "ping of death", where the amount of ping traffic is enough to disrupt their connectivity to the internet.

In more common scenarios, the amount of available upstream bandwidth becomes saturated by repeated requests for a file on the target computer, or by large UDP packets.
When the attack comes from more than one source IP, it is known as a distributed denial of service attack or DDOS

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