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Suppose I have a router with external ip address as 123.123.123.123 and within that router I have 5 devices connected one of them being a server which has been assigned a private ip address as 192.168.1.1.
One important thing is that the server on my private network is running multiple services such as HTTP at port 80, ftp at 21, mail at 60,etc.

With that now I've port forwarded my router so that external any request to ip address 123.123.123.123 on port 900(any arbitrary port chosen by me) would redirect the request to my server at 192.168.1.1 thats hosting multiple services.
Now imagine that a distant away user types in address 123.123.123.123:900 in the address bar of his browser. So the question is that which service of the server will be invoked as no port number regarding the server's service has been mentioned.

Kindly help .

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    In port forwarding, you forward a port to a port. So, you need to choose to which port on the local interface you forward the port 900 on the external interface. You can't forward from one port to multiple ports, unless you are able to tell the firewall from some other information than the port number to forward the request to.
    – zagrimsan
    Dec 1 '15 at 10:40
  • In port forwarding, you forward a port to a port. Really ?. Because where ever I read about port forwarding it stated that you actually map and internal ip address to a port number Dec 1 '15 at 10:42
  • Please do quote/link one such source.
    – zagrimsan
    Dec 1 '15 at 10:44
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    Let us continue this discussion in chat.
    – zagrimsan
    Dec 1 '15 at 10:54
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    On that server which service will be called ? Only the one listening on port 900. On any given port, you can only have oneservice listening. That's why you must forward port 901 to port 5550, port 902 to port 7765, and so on:**one to one**. Dec 1 '15 at 11:27
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When you use port forwarding you have to declare the outside port and inside port.

You can match for example outside port 900 to inside port 80.

If you simply open port 900 on the router, and don't configure the port on the server side,then most likely will be the same port number.

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