Think of it as an Internet traffic warden.
Port forwarding is a way to ensure that traffic from the Internet is passed to the correct device on your home network.
There are many reasons that you might want to ensure this, a lot of games require that certain ports are forwarded in order that they function properly when playing online.
How IP addressed work
Think of it this way, your computer has a local IP adress, something like the following:
This is the address given to your computer by your home router so that other devices on your network can identify it. The first two groups of numbers are usually always 192.168 but the rest can change depending on your router. This is called a private IP address.
In a much broader sense, your router has an IP address which is given to it by your Internet provider. It’s slightly different here as there are a lot more routers on the Internet than there are computers on your network, so each of the address parts could be between 0 and 255, as below:
Your routers IP address is known as a public IP address.
Each IP address, private or public, also has a serious of ports associated with it. This is so that different kinds of traffic can be properly identified by the router and by your computer.
Ports are described by putting a colon (:) after the IP address and then entering the port number, for example:
Port 80 is traditionally the port used for normal website traffic, so you don’t normally have to enter it. There are other ports commonly in use as well, such as 21 for FTP and 5000 for UPnP. A full list of know port uses can be found here.
So, what’s port forwarding then?
Let’s assume that a game or some other service uses port 5005.
In order for your computer to be connected with that game or service you’d need to make sure that all traffic coming into your home router from the Internet on that port number is sent to your computer.
Port forwarding allows this to happen. It’s a setting on your router that will forward all traffic on port 5005 directly to your PC’s private IP address.
All other ports (other than the ones most commonly used) will be blocked by your router in order to prevent unauthorised access to your PC, just like below:
How to actually achieve this will depend on the exact model of router that you own, but most will have a table which will allow you to put in an incoming port number and specify a private IP address that incoming connections will be sent to.
You can access your router by following the instructions left by your ISP when they installed it, if they didn’t leave you any information on how to access your router, give them a call and ask for instructions on how to set up port forwarding.