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?

Answer 1

(* Its an old question and the answers are okay, just one addition)
its called IP spoofing, firewalls are working hard to detect and block it and network-bridges might get confused by it (for a while)
http://en.wikipedia.org/wiki/IP_address_spoofing