"Remote port forwarding" forwards traffic coming from the Internet to our VPN server ports to a specified local port of your client.
By default, your account has no forwarded ports, and this is good as long as you don't wish to have a service reachable from the Internet. For example, suppose that you want to run a web server behind our VPN, or that you wish to receive incoming connections to your BitTorrent client in order to improve p2p performance, or to seed a file. Without at least one remotely forwarded port, your service could not be reached from the outside, because our VPN server would reject the proper packets to your service.
Usually this is a good security measure against attacks, but it prevents your services to be reached from the Internet.
When you remotely forward a port, our servers will open that port (TCP, UDP or both, according to your selection) and will properly forward incoming packets to you on that port. The service will be reachable from the exit-IP address of the VPN server your system is connected to.
You can forward up to 20 ports simultaneously. You can do that on our website, in your account "Client Area". You can't forward ports lower than 2048. You can map a remotely forwarded port to a different local port: this is useful for a variety of cases, for example when your service listens to a port lower than 2048 or when the port is already reserved.
IMPORTANT: do NOT forward on your router the same ports you use on your listening services while connected to the VPN. Doing so exposes your system to correlation attacks and potentially causes unencrypted packets to be sent outside the tunnel from your client. However, if you connect a router (for example DD-WRT, Tomato based firmware router) an additional step is required, please see https://airvpn.org/topic/9270-how-to-forward-ports-in-dd-wrt-tomato-with-iptables/
NOTE: you can't reach your listening service(s) through the VPN server exit-IP address from the very same machine that's running it/them and is connected to a VPN server, or from any other machine connected to that same VPN server.