Setting Up The Nat Function - Siemens Gigaset SE361 WLAN User Manual

Setting up the NAT function

Your Gigaset SE361 WLAN comes provided with the NAT (Network Address Translation)
function. With address translation, several users on your local network can access the
Internet via one or more public IP addresses. In the default setting, all local IP addresses
are mapped to your router's public IP address.
One property 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 on 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 on the local network and 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 specific pro-
tocol (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 on
the local network. Initially, these applications will not work if Network Address Transla-
tion (NAT) is activated.
Using port forwarding (the forwarding of requests to specific ports) you make the router
forward requests from the Internet for a certain service, e.g 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 SE361 WLAN forwards data from the set port block to the PC which has
previously sent data to the Internet via a certain port (trigger port). This means that per-
mission for data transfer is not tied to one specific PC in your network, but only 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. To this trigger port you then assign the public ports
that have to be opened for the application.
u
The router checks all outgoing data for port number and protocol. If it recognises a
match of port and protocol to 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, it allows the data through and routes
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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