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Page 102 NB1300Plus4 ADSL Modem
■ NAT is designed to share one Public IP address with many Private IP addresses.
■ Most common NAT is actually Dynamic NAPT - "Dynamic Network Address Port
Translation".
■ 'Dyn NAPT' chooses random ports on the WAN to allow communication with the LAN.
■ Dyn NAPT by its nature is a kind of passive firewall. It hides internal computers but does
not react to an attack.
NAT is enabled by default in most Home / SMB routers as 99% of their intended use is in this
mode of operation.
What is Port Forwarding?
Port Forwarding (sometimes called Virtual Server) is used to overcome the restrictions of NAT.
Most Internet services are linked to a specific port (e.g. FTP = port 21, WEB/HTTP = port 80),
so you can make reservations in your NAT table that fix an external (public) port to an Internal
(Private) IP address (Computer). These reservations are called 'Port Forwards' and they mean
that any inbound Internet request to open that port will be forwarded to the IP address
specified. The advantage of port forwards is that they allow you to host Web servers, Game
servers and run more specialised Internet applications (P2P programs) through your NAT
Router / Firewall.
To configure Port forwarding in most routers you need to know the following information:
■ The Public Port or ports to forward this is a number between 1 and 65535
■ The Protocol used (usually TCP or UDP, sometimes both)
■ The Internal IP address of the computer that is running the Service (e.g. Web service)
■ The Private Port number that the service is running on (often the same as the Public port,
not always req'd)
For example, if you are using a NetComm NB1300 ADSL router and you use a simple web
server (port 80) such as Abyss Web server (http://www.aprelium.com) on your Computer with
an IP address of 192.168.1.10;
Then your Port forward rule for this might look like;
Public port: 80
Private port: 80
Protocol: TCP (or both)
Private IP: 192.168.1.10
Note: Port forwards should only point to computers with statically assigned IP addresses,
otherwise the data might get directed to the right IP address but the wrong
computer. If you want to be able to launch your Special Internet Application from
any computer or IP address you should use the Universal Plug and Play (UPnP) or
"Port Triggering" (sometimes known as "Special Application") functions of your
router if it has them.