Spoofing an IP Datagram
IP packets are used in applications that use the Internet as their communications medium. Usually they are generated automatically for the user, behind the scenes; the user just sees the information exchange in the application. These IP packets have the proper source and destination addresses for reliable exchange of data between two applications. The IP stack in the operating system takes care of the header for the IP datagram. However, you can override this function by inserting a custom header and informing the operating system that the packet does not need any headers. You can use raw sockets in UNIX-like systems to send spoofed IP datagrams, and you can use packet drivers such as WinPcap on Windows . Some socket programming knowledge is enough to write a program for generating crafted IP packets. You can insert any kind of header, so, for example, you can also create Transmission Control Protocol (TCP) headers. If you do not want to program or have no knowledge of programming, you can use tools such as hping, sendip, and others that are available for free on the Internet, with very detailed documentation to craft any kind of packet. Most of the time, you can send a spoofed address IP packet with just a one-line command.
Why Spoof the IP Source Address?
What is the advantage of sending a spoofed packet? It is that the sender has some kind of malicious intention and does not want to be identified. You can use the source address in the header of an IP datagram to trace the sender's location. Most systems keep logs of Internet activity, so if attackers want to hide their identity, they need to change the source address. The host receiving the spoofed packet responds to the spoofed address, so the attacker receives no reply back from the victim host. But if the spoofed address belongs to a host on the same subnet as the attacker, then the attacker can "sniff" the reply. You can use IP spoofing for several purposes; for some scenarios an attacker might want to inspect the response from the target victim (called "nonblind spoofing"), whereas in other cases the attacker might not care (blind spoofing). Following is a discussion about reasons to spoof an IP packet.
Guilty 
