Creating a Server WITHOUT port fowarding

Question

How do I make it so that a person can download the server.jar file, run it, and have people connect to it (socket 11114) without having to configure their router to foward that port. I know its possible because in professional games, people can connect to each other and the user doesnt have to use port fowarding.
Thanks

Answer 1

Normally a client doesn't need to forward ports to connect to a server at all. A router usually prevents an arbitrary connection to a computer it's routing, since it may be routing many computers and the intended one may be impossible to identify unless pre-configured with the router. Connecting from the routed computer does not have this problem, so long as the server has its ports ready, except for the rare case where the router doubles as an anti-trojan firewall, in which case it needs to be configured accordingly, but that's really the client's problem(and fault, for enabling such a feature on the router in the first place, or actually picking out the extra-rare router that has such a feature on by default).
This simply requires that the client connects to the server, and never vice versa. This is okay for a connection-based protocol, like TCP.
For a datagram protocol like UDP, most modern routers tend to "remember" where routed computers sent a packet to, and use it to figure out where packets sent back should go, so as long as the client sends the first packet, and so long as the server sends replies within a certain timelimit(usually 60 seconds), it should work without any special port forwarding.
The few users using a router without that support would either have to configure their router, or use TCP only.