IP spoofing 
A technique that hackers use to gain access to computers or networks. The hacker uses a variety of techniques to find an IP address from an external computer that is allowed access to the target network or computer, or to access an IP address from the addresses that the computers on a targeted network use. The illictly obtained IP address is then used to modify the packet headers that the hacker sends, thereby tricking the target network or computer into allowing the hacker access.