 <p>Utterly, utterly impossible. You won't even be able to open a TCP connection because the other website's server will try to handshake with the client, and fail.</p> <p>An IP address isn't just any old ID, it's the actually <em>address</em> that servers will send any response to. Spoofing it basically only makes sense if you can fit your request into a single IP packet (which rules out TCP and thus HTTP) and are not interested in the response. Even then it can fail because your ISP's routers may have anti-spoofing rules that drop packets with "outside" IP addresses originating from "inside" networks.</p>