Setting Up The Nat Function - Siemens Gigaset SX763 WLAN dsl Manual

Configuring Advanced Settings

Setting up the NAT function

The Gigaset SX763 WLAN dsl comes equipped with the NAT (Network Address Transla-
tion) function. With address mapping, several users in the local network can access the
Internet via one or more public IP addresses. All the local IP addresses are assigned to
the router's public IP address by default.
One of the characteristics of NAT is that data from the Internet is not allowed into the
local network unless it has been explicitly requested by one of the PCs in the network.
Most Internet applications can run behind the NAT firewall without any problems. For
example, if you request Internet pages 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 precisely one port for the application. A port in this con-
text is an internal PC address, via which the data is exchanged between the Internet and
a client on a PC in the local network. Communicating via a port is subject to the rules of
a particular protocol (TCP or UDP).
If an external application tries to send a call to a PC in 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 users in the
local network. These applications cannot be run if Network Address Translation (NAT)
has been activated.
Using port forwarding (the forwarding of requests to particular ports) the router is
forced to send requests from the Internet for a certain service, for example a game, to
the appropriate port(s) on the PC on which the game is running.
Port triggering is a special variant of port forwarding. Unlike port forwarding, the
Gigaset SX763 WLAN dsl forwards the data from the port block to the PC which has pre-
viously 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 the network, rather to the
port numbers of the required Internet service.
Where configuration is concerned, this means:
You have to define a so-called trigger port for the application and also the protocol
u
(TCP or UDP) that this port uses. You then assign the public ports that are to be
opened for the application to this trigger port.
The router checks all outgoing data for the port number and protocol. If it identifies
u
a match of port and protocol for a defined trigger port, then it will open the assigned
public ports and notes the IP address of the PC that sent the data. If data comes back
from the Internet via one of these public ports, the router allows it through and
directs it to the appropriate PC. A trigger event always comes from a PC within the
local network. If a trigger port is addressed from outside, the router simply ignores it.
76