What is a port ?

~ A port is a logical connection used by the programs and services to exchange information.

How the connection is established between two machines ?

~ A connection is established when a request is initiated to the destination's public IP using a specific port which eventually reaches the router .

What is the function of a router ?

~ A router forwards the connection request from the internet to the destination by using the port forwarding mechanism without which a router will not be able to figure out where to send the request.

What is port forwarding?

~ It is telling your router to accept connection request from specific ports . For example , port 3389 in the case of a remote desktop connection in windows. Once the router is configured with this rule then any request with port defined will be allowed to connect.