An open port on the internet is a service that allows other computers on the internet to connect to it. In a home networking environment, an open port is typically the result of a process known as Port Forwarding. Port forwarding is often misunderstood – the Port Forwarding article on Wikipedia says:
Port forwarding or port mapping is a name given to the combined technique of
- translating the address or port number of a packet to a new destination
- possibly accepting such packet(s) in a packet filter (firewall)
- forwarding the packet according to the routing table.
The destination may be a predetermined network port (assuming protocols like TCP and UDP, though the process is not limited to these) on a host within a NAT-masqueraded, typically private network, based on the port number on which it was received at the gateway from the originating host.
Imagine that you have a house. You’re concerned with security, so you build a big electrified fence all around your house to keep the bad guys out. That’s kind of what your router’s firewall does. The front door to the house is pretty heavy with big locks – also to keep the bad guys out. With all of that security, you’d feel pretty safe sitting in an interior room of the house with the door locked, right? That room is sort of like a computer on your network that’s being well protected by the router.
Now imagine that your friends would like to be able to get into your room without having to go through all of the trouble of getting through security. In a fit of madness you start digging a tunnel through the floor of your room under the house, and have it come out of the ground outside of the electric fence. Problem solved, right?
That’s exactly what port forwarding a router does. It allows people from outside of your network to connect to computers that would otherwise be impossible to get to. Obviously, there are huge security implications if this is done.
So why do this? The reason is that often there are software applications running on those computers (often these are games) that require port forwarding to take place. Rarely are the implications of doing this explained to the users, often with not-so-good results.
Examples of games that require port forwarding for some features to be used include:
- Call of Duty
- World of Warcraft
A more complete list can be found here.