And oh, IP Spoofing helps too. If you dont know what IP Spoofing is..
IP Spoofing is a technique used to gain unauthorized access to computers, whereby the intruder sends messages to a computer with an IP address indicating that the message is coming from a trusted host. To engage in IP spoofing, a hacker/cracker must first use a variety of techniques to find an IP address of a trusted host and then modify the packet headers so that it appears that the packets are coming from that host.

The packets sent from one computer to another has IP Addresses on them, one is the IP address of the source of the packets, and the other is the IP address of the destination of the packets (just like any regular postal mail). You can modify those packets with a packet annalyzer to change the IP addresses of those packets.