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1. The following diagram illustrates a basic, two-PC network connected to the Internet through a NetComm NB1300 router/gateway [so called because it is a 'gateway' to the Internet for all the network computers.]
The EXTERNAL or PUBLIC IP address has been allocated to the router by the ISP; the INTERNAL or PRIVATE IP addresses have been allocated by the router to the PCs on the network. In this example, the process of allocating IP addresses has been handled automatically by DHCP, or Dynamic Host Configuration Protocol. The router has both an Internal and an External IP address, and translates Web requests between these two address sets. So when the user of PC1 clicks on a Web Address, the Router tracks this request from that PC, and when the Web page comes back, routes it to the PC [= the IP address] that requested it.
This process is called 'Network Address Translation.' It is a very important point to get about broadband modems and has to be understood for discussion of 'port forwarding' to make sense.
2. Setting a Port Forward
So far, the example has dealt with the 'default configuration' which will handle web pages and email without further configuration.
However say you want external users to be able to get files from PC1 across the Internet by FTP. To do this, the user will attempt to navigate to the file location by entering an FTP address, such as ftp://204.55.165.195.
When the FTP requests reaches your Internet Gateway, it will attempt to connect through port 21, the common port for FTP requests. But in this case the request will simply be dropped by the router. Why? Because as far as the Router/Gateway is concerned, no-one on its network has requested this information, and so there is nowhere to send it. There is no corresponding NAT entry for this request, meaning 'the port is not open'. The requesting user will get an error message saying 'ftp session terminated'.
So you have to set up a 'Port Forward Rule' to enable this function. This will tell the router, 'When an FTP request is received by 204.55.165.195, then direct this request to port 21 on the PC with IP Address 192.168.1.2.'
Let's look at how this is configured on an NB1300:
Here you have entered a port-forwarding rule which tells the router 'When a request is received for the 'public' (or 'outside') Port 21, direct it to 'private' (or 'inside') port 21 on the PC with IP address 192.168.1.2.
After entering these values, click on 'add this setting' to save the port forward in your configuration table:
This will add the 'Port Forward' to the table in the router which tells it how to handle incoming FTP requests.
There is plenty more information about port forwarding in all the NetComm User Manuals.
The next page provides a summary of some of the main components of Port Fowarding and some useful links. Page 1, 2, 3
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