Setting Up The Nat Function - Siemens Gigaset SX541 WLAN dsl Manual

Setting up the NAT Function

Your Gigaset SX541 WLAN dsl comes provided with the NAT (Network Address Transla-
tion) function. With Address Mapping, several users in your local network can access the
Internet via one or more public IP addresses. In the default setting, all the local IP
address are mapped to your router's public IP address.
One of the characteristics of NAT is that data from the Internet is not allowed into your
local network unless it has been explicitly requested by one of the PCs in that network.
Most Internet applications run behind the NAT firewall without any problems. If you
request Internet pages, for example, or send and receive e-mails, the request for data
from the Internet comes from a PC in the local network and so the router allows the data
through. The router opens exactly one port for the application. A port is an internal PC
address through which the data is exchanged between a server on the Internet and a
client on a PC in the local network. Communicating via a port follows the rules of a par-
ticular protocol (TCP or UDP).
If an external application tries to send a call to a PC within the local network, the router
will block it. There is no open port via which the data could enter the local network.
Some applications, such as games on the Internet, require several links, i.e. several ports
so that the players can communicate with each other. In addition, these applications
must also be permitted to send requests from other users on the Internet to the user in
the local network. These applications cannot work if Network Address Translation (NAT)
has been activated.
Using port forwarding (the forwarding of requests to particular ports) the router is
forced to forward requests from the Internet for a certain service, fro example a game,
to the appropriate port or ports on the PC on which the game is running.
Port triggering is a specific variant of port forwarding. Unlike port forwarding, in this
case the Gigaset SX541 WLAN dsl forwards data from the set port block to the PC that
has previously sent data to the Internet via a certain port (trigger port). This means that
approval for the data transfer is not tied to one specific PC in your network, rather to the
port numbers of the required Internet service.
Where configuration is concerned, this means:
u
You have to define a so-called trigger port for the application and also the protocol
(TCP or UDP) that this port uses. You then assign the public ports that have to be
opened for the application to this trigger port.
u
The router checks all outgoing data for the port number and protocol. If it recognises
a match of port and protocol to a defined trigger port, it will open the assigned pub-
lic ports and note the IP address of the PC that sent the data. If data comes back from
the Internet via one of these public ports, it allows the data through and directs it to
the right PC. A trigger event always comes from a PC within the local network. If a
trigger port is addressed from outside, it is simply ignored by the router.
Configuring the Advanced Settings
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