Define Port Forwarding

To access a service from the internet which is running behind on our router, in that case, port forwarding is required. Port forwarding is the process of forwarding traffic to a specific destination which is originated from the internet or outside of the local network.

Suppose you have a service that is running on your local network is accessible only for your local user. Now you want to publish that service on the internet so that any internet user can access this service from anywhere in the world. Internet users will submit a request on the router to reach a specific service using that service's port number. Then the router will check its forwarding table if any entry is available for that port number. Every router maintains a port forwarding table where every service (port number) should be listed that are intended to access from the internet. Not only the service name, which computer is providing that service also be listed. And that's why a router can redirect the traffic to a specific destination what is an internet user searching for. This process is working like "Destination NAT" or "PAT (Port Address Translator)".

Assume that, we have an FTP server running on our local network. Only our LAN user can access this FTP site. No internet users can't access because this site is not published on the internet. Now we want to make available this FTP server on the internet. We have two options to accomplish this task. The first one is, we can use one public IP address directly in our FTP server and that's why it will automatically available on the internet. And the other one is, we can forward the FTP service request to our internal FTP server from our router, which is called port forwarding.