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Forwarding a port is the process of setting a translation from port A to port B, over two IP addresses (eventually, the same host). This is generally used to open a local area network to the Internet, through its network gateway. This allows hosts from the WAN to connect services from a LAN, hidden behind their router.

Port forwarding tables

Port forwarding settings are usually gathered into a forwarding table, saved by the network gateway/router. This table contains the following parameters from each forwarding rule :

  • Source host : receives the packets and forwards them, usually the router.
  • Destination host : new destination set for packets while forwarding.
  • Source port : port opened and dedicated to packets forwarding on the source host.
  • Destination port : port listening on the local host, and processing the forwarded packets.

On some more recent hardware, it is possible to set rules for a range of ports, and not for a unique port. This allows the translation from ports A to N to the same ports on the local host receiving the forwarded packets. This is useful if your processing application requires several (and consecutive) ports to work.

Example : HTTP forwarding

Consider the following wireless network :

  • Router is located on 192.168.1.1, and available on the Internet through IP 1.2.3.4.
  • The web server is located on 192.168.1.2 and listens on ports 80 (HTTP) and 443 (HTTPs).

In this case, the forwarding consists in the translation from 1.2.3.4:80 (available from the WAN) to 192.168.1.2:80 (available from the LAN only). This configuration takes place on the forwarding host, i.e. the router on 192.168.1.1. When receiving packets on port 80, it'll automatically forward them to 192.168.1.2:80, which will process the HTTP request, and send its response through the same but opposite network route (web server - router - client). Once this configuration is complete, it becomes possible to access the local web server from the WAN, by requesting http://1.2.3.4/

An identical configuration would also be made, translating 1.2.3.4:443 to 192.168.1.2:443.

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