IP spoofing and routers, modems

[PacketGuy]

I've been messing with ip spoofing recently and got it working in my local network. Before I try this with actual game servers, I'd like to clear things up. How is router going to handle packet with spoofed ip when it passes it to the global network? Will it just pass it through like any other packet or take some action? Will spoofed ip go all the way through router, modem, ISP to destination unchanged?

[PacketGuy]

I did some testing but I'm still stuck. I crafted a packet, from ethernet header to payload data, but ip just won't change.

My problem at the moment is that no matter what I send to my modem, it always seem to transform spoofed ip into my real ip. At remote destination I always get reports that message was sent from my real ip. I guess I have to go to router options, but I have no idea what to do there.

Another thing is that modem seem to act like a router as well. Even though I connected my computer to modem directly by cable, ipconfig command shows that my computer is still part of subnetwork 192.X.X.X.

So, how do I force my modem to stop changing ip address of the packet I created and instead just forward it further into network?