Microsoft Certified Systems Engineer (MCSE) Question:

IP spoofing is a mechanism used by attackers to gain unauthorized access to a system. Here, the intruder sends messages to a computer with an IP address indicating that the message is coming from a trusted host. This is done by forging the header so it contains a different address and make it appear that the packet was sent by a different machine.

Prevention:-
Packet filtering: - to allow packets with recognized formats to enter the network
Using special routers and firewalls.
Encrypting the session