for a system directly coupled to the net, you need a service running on your
system, listening on the desired port.
for any system behind a router, the router must port forward the specific
port from the WAN side, to the IP-address of the system with the active listener
as most LANs use DHCP, port forwarding will need a static ip assignment OR
a mapping of the MAC to the IP-address to be give via DHCP.