Is port forwarding possible in C # / software? Isn't it controlled only by a router?

Question

Is port forwarding managed by a router? For example, I was looking for some software applications that seem to be moving forward with great success, but technically this seems impossible. The packet must first go to the router. The router should redirect it to the correct computer based on port forwarding rules. So, how can an application manage port forwarding if the package should go to the computer on which this application is running, first of all, which means that port forwarding is already successful in the first place?

Answer 1

You see UPNP , which allows a program running on a local network to forward a port on a router.