What Is Port Forwarding?
How many times have your heard the phrase 'port forwarding'? How many times have you asked yourself what port forwarding is? Before understanding why and how to port forward, it is important to understand what an Internet IP is vs. what a NAT (Local) IP is.
Think of NAT as a phone extension. Your office may only have one phone number, but many phones tied to that single phone number have extensions. The default phone, your receptionist, answers general requests, and this is what your router does.
What Is A Port?
It is equally important to understand ports. The ports used in port forwarding are not physical ports that you will find on the back of your computer. These are virtual ports that exist as part of the TCP/IP and UDP protocols. There are 65536 ports ranging from 0-65535. The lower-numbered ports are most commonly used.
I'll use Windows Remote Desktop Connection as an example (also known as Terminal Services).
Terminal Services listens (by default) on port 3389. When opening the Remote Desktop Client (run command "mstsc") and typing in an IP address, the client will attempt to connect to that IP on port 3389.
Entering an Internet IP, unless your computer is connected directly to a cable modem without NAT, this should all work just fine and dandy.
How It Works
When you are behind a router or a modem that is using NAT, you need port forwarding to get it to the local IP address.
Simply put, the router will receive the connection, but isn't equipped to handle that request without instructions. The router needs to know which machine on your Local Area Network (LAN) to send the request. Without a port forward instruction in place, the request will fail.
Obviously to some, not to others, the router can only port forward this request to one PC at a time. So, if you are running more than one machine, you will need to change the default port and forward it as necessary, or simply use a VPN.
If you do not know how to port forward with your router, there are usually instructions on the manufacturer's website with pictures.