So i can Connect a DOSBOX where Carmageddon runs, with a PC where Carmageddon runs on real DOS?
Yes, that is the main idea. But I can't say it for sure about Carmageddon, so far I only tested it with Duke Nukem 3D.
Is there a Windows Version too?
No. I am not familiar with Windows network API and I am not using Windows anyway, so there will be no Windows version from me for sure.
Or can I use another PC as a Router between the Dosbox and the Real PC?
Since ipxbox can serve only one UDP client it is not really useful as a remote service so it is the reason why it binds its UDP socket to a loopback address 127.0.0.1. This means that you can't use it as a router. But if you really want you just need to change one line in the code (0x7F000001 => INADDR_ANY) and you (as well as any bad guy in your network ;-) should be able to use a Linux host connected to a L2 IPX network that runs ipxbox as a router for a DOSBox that runs on a Windows host.