Why is it not possible to fake an IP address?

Question

I know it's possible to change an IP address using a proxy, but I'm interested in why is it not possible to change it by sending changed data from a single computer?

Update:

Thanks for quick replies, but it's not clear, what would happen, should a stateless protocol be used, like HTTP. I'm not familiar with lower-level network communication, but it seems that theoretically it should be possible to faken IP address if a PC connects to the server and receives the reply immediately.

Answer 1

It is fully possible to send data with a fake sender IP. You'll have a hard time getting replies though, since the replies will be sent to the fake IP and never reach you. Additionally, if you send data with a "from" IP that a router doesn't expect to arrive from you, the router will often simply drop it, and sometimes inform the ISP that you're up to something that violates your TOS.

EDIT: In response to edited post.

Any protocol (such as HTTP) that is built on top of a connection-oriented protocol (TCP) will require a valid IP. The cases where fake IPs have been prevalent mostly involve Denial Of Service attacks, using UDP, ICMP or the initial SYN packet in a TCP handshake.

Answer 2

You can use any IP address you want for sending, but replies will go to the IP address you used, so you cannot establish two-way communication this way.