DDOS attacks can be done using just one computer with a fair amount of bandwidth.
Most of the time a DDOS attacker uses several to many computers that have been exploited by the hacker to attack a target in unison, the main PC would be a Master that tells the other PC's when and how to attack, these computers are called slaves or zombies.
DDOS is a basic flooding program that sends out a packet repeatedly to the target ip address that ovewhelms the target server.
now how do I know you can use a single PC you might ask, well in a server service environment we test the vulnerabilities of servers using programs that perform various tests including flooding the server and that helps to show how to better protect a customers server, we use a single computer for the flood testing.

"... Shh! Be vewy qwiet! I'm hunting wuntime ewwows!"

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