If you have a router at home or your office, you will need to forward ports in order for outside traffic to get into your network. Think of your router as being a huge electric fence or wall, with a few doors or openings. This electric fence or wall serves as your barrier and security blanket from the scary outside Internet world. Your router comes pre configured with a few of those doors (or ports) open to let you access the internet, but the others are closed tight. So, in order to run a mail server, game server, access your computer remotely, etc you will need to open an extra door or two in your router in order for the outside traffic to get inside. This is called Port Forwarding.
Port forwarding: Also referred to as port mapping, is a method of forwarding a network port from one network node to another. This technique can allow an external user to reach a port on a private IP address (inside a LAN) from the outside using a NAT-enabled router.
Port forwarding allows remote computers (e.g. public machines on the Internet) to connect to a specific computer within a private LAN.
- forwarding of port 80 to run an HTTP webserver within private LAN from internet
- forwarding of port 22 to allow Secure Shell access within private LAN from internet
- forwarding of port 21 to allow FTP access within private LAN from internet
Port forwarding is not necessary with IPv6, because every IPv6-enabled device has a public IPv6 address.