More troublesome, sophisticated computer users can "spoof" IP addresses, or use one assigned to somebody else. They use a simple piece of software to forge the IP address on packets of information sent from their computer, much like someone who puts an address on the back of an envelope that isn't theirs. The people most likely to spoof are the very tech-savvy youngsters also mostly likely to be stealing music. Even if the RIAA had an IP address it believed belonged to Andersen, Lybeck thought, that wasn't necessarily the case.
An otherwise good article got it wrong here.
Sure you can place a forged source address in an IP packet that you send. But no conversation can take place because the Internet "knows" a route to that forged address (which isn't your machine) and it is not possible to change that.