Some ports are assigned, like email, telnet, remote assistance and web browsing. For instance, web browsing typically uses port 80.
Lets say you have 5 computers at home, all behind one router. That router has your only real IP address. Each computer has a number assigned to it by the router which is it's internal (non internet routable!) IP address. When you want to play a poorly coded game and it needs access on port 3874 (for instance) you can tell the router to forward *ALL* data coming into the router to ONE particular computer/IP address.
You cannot tell a router to port forward one particular port to more than one computer/IP address.
Software written today should all be fully tested and router compatible. People claimed that you needed to forward a handful of ports to play BFBC2 but I have 3 machines here that all play it at once. If I had port forwarded to one computer then I would have been limited to playing on one computer. Regardless, port forwarding is not needed with BFBC2
I hope my description wasn't too basic and kinda gave you the idea...
You internal network is controlled by the router. This is why you get a 192.168 address as it is a special IP address used for internal networks. This allows each computer in your network to easily communicate with each other over using the router. In addition to providing some routing capabilities the router also talks to the outside world. Now when you are reciving information from the internet on a specific port the router needs to know where to send that information. Information coming from the outside source usually only knows your public IP address not the internal 192.168 address. So you tell the router that all information coming to it on Port XX should be sent to computer 192.168.x.x . It really shouldn't be needed with most modern games. It usually applies to things like hosting a web server, utorrent, and other programs where you are accepting incoming connections from multiple external IP's.
I didn't mention the IP address range simply because I've run into routers that use the 10.x.x.x range instead. Murphy's law dictates that if I mentioned 192.168.x.x then it would be virtually guaranteed that he would have the other.
Whenever a program wants to "listen" for incoming connections over the internet, it needs to open a "port". The port is a 16 bits integer, so it has a range of 0-65535.
Port forwarding is a simple concept : since your home PCs are behind a router, and the router is the only device directly connected to the internet, it's the one that has to open the port for listening. So for example, if you have a web server (standard port 80) behind a router, only the local computers connected to the router will be able to access it. To enable anyone from the internet to access the server, you would create a port forwarding on your router, so that it redirects (forwards) all incoming internet connections on port 80 to your specified server.
Some games need a port open for listening to incoming connections, especially when they are the kind where you can host a game. You basically need to tell the router to open these ports for listening and forward the traffic to your local PC.
I can be a bit long-winded, but I hope that helped.
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