IP Address Spoofing

The protocol that is generally used to communicate between the systems is Internet Protocol (IP).
The data is sent through the internet in the form of packets. Each packet has a header which contains general information about the packet. The header of the packet in the IP contains the source address and the destination address. The source address is generally the IP address of the system from where the packet is sent over the internet and the destination address is the IP address of the system to which the data is sent. In IP address spoofing the source address in the header is replaced by a false address and is sent to the target system. The responce from the target system is sent to the false address. The attacker may be able to predict the responce from the target machine or he can direct the responce to his IP address.
The IP spoofing is usually done in Denial of Service (DoS) attack. Here the attacker doesn't need to know the responce of the target machine. He need just to sent the packets to the target with false address. Each packet to the target may be fixed with diferent false source address. So it is difficult to filter the unnecessary packets.
It is difficult for the attacker for attacking a system which requires authentication, but it is possible to attack the target to some extend. In some networks for example in the case of a network in the bank every system is interconnected and it may not require authentication to communicate between these systems. If the attacker wins in gaining access to one of the system in the bank, he can simply attack the whole network.
One of the method to prevent spoofing is to filter the incomming and the outgoing packets. The gateway to a network usually perfoms ingress filtering, which will prevent the data comming from the outside network with source address within the network. Similarly the gateway performs engress filtering which prevents the packets with source address outside the network. These measures prevents the spoofing only to some extend.