Malicious intruders use literally hundreds of methods and tools when they attempt to compromise PCs. The following are some the most common attacks:

Port scan. To attack a PC, an intruder must determine which TCP or UDP ports (and thus services and programs) are open on that computer. Each program that connects to or accepts connections from the Internet is assigned an IP port number (0 to 65536). Port numbers often indicate a particular type of service. For example, Internet email ...