Consider we have port 65434 open on router (open for WAN). The port forwarding is done to a host which does not exist. Can a hacker exploit this open port on router which leads nowhere?
Depending on how your network is configured a hacker might be able to in a phishing email.
So pretend hack sends you an email which you open by accident. They now have control of your computer.
- They can now change your IP to the non-existent one, and have a direct way in and out of your network.
- Setup a virtual PC with said IP address and communicate with their servers.
Also if it does have an IP, it should be on a different subnet to further mitigate the risks. Otherwise attacks 1 & 2 above are even easier.
You didn't specify exactly how your router was configured to forward. If it is simply based on IP, and the hacker can place a virtual PC on your network with that IP game over.
If your forwarding to a specific network adapter say eth5 then it doesn't care about the IP.
Either way your consuming routing resources unnecessarily, instead of just dropping the packets.