Both. There is special software designed to do just that, and in essence here is what it does. It sends a lot of packets, the "thing" that contains data, really fast. Most web-servers that are for large corporations, such as sprint, Steam, Activision, ect. Have some sort of "defense" against this. What this defense does is when it gets a large number of packets from an IP address at one time, it automaticaly blocks that IP. This is effective against someone like I mentioned in my previous post, they don't know what they are doing. Say, for instance, you have a significantly large botnet. (A single unit in a botnet is usually a "zombie" computer that a certain host has control of, and most of the time the person that owns this "zombie" computer, won't even realize someone else has control of it. Now imagine that someone has a botnet in the upper 1000's.) Most web-servers, or other computer related targets, won't be able to handle such a large DoS attack. Also, no, they usually do not just randomly select IP's, that gets a little risky. The FBI's cyber-crime unit will not usually go after a "hacker" who is DoSing unless he/she does up to $5000 worth of damage, or he/she attacks a government facility, such as a school website, government website, ect. A DoS attack is only effective if you have an outside source, or multiple outside sources. A home computer cannot sustain a DoS attack for a prolonged period of time, for a few reasons. It eats up your bandwidth, your ISP usually monitors high bandwidth usage, and if yours shoots up in a very short amount of time, you are either downloading a very large file, or you are DoSing. There are a lot more intricate things to "hacking" but in reality, those would be the basics. Next up would probably be port scanning, you basically scan an IP address for open ports, from which you can do whatever with.