IP address spoofing - Definition from the Hotspot Shield Glossary

The process of IP Address Spoofing has the sole purpose of hiding a user’s own IP address and identity or even impersonating another device/address. The actual process changes the IP protocol header data - This header data contains important information, and more specifically, the senders IP address.

To show a different IP address, this header information will be changed so the receiving party thinks the data is coming from another source. One important thing to note is that when data is received from a spoofed IP address, a response from the server for example will usually be sent back to that spoof address, therefore, people using IP spoofing should not care about receiving a response. Owing to this fact, IP spoofing is generally used for illegitimate reasons such as DoS attacks.