Well no, IP addresses are not copyrighted. But if a man-in-the-middle injects packets into my TCP/IP streams with another party, putting my IP address into them or my sequence numbers into them, and it’s against my wishes and knowingly against my wishes, I’m not sure why forgery isn’t a good term for that. They are pretending to be me to the other side. This is quite different from sending such packets when a link is going down for reasons beyond my control. In that case, I want the other side informed that the link is down, and as such one can reasonably argue I authorized or would have authorized the MITM to use my identifier in a packet. When you use somebody else’s identity when they don’t approve it, that’s forgery.