Siemens Simens Gigaset WLAN dsl/cable SE551 - Manual (Page 61)

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Configuring the Advanced Settings
Gigaset SE551 WLAN dsl/cable / ENG / A31008-M1029-B101-2x-7619 / configure_router.fm / 03.11.2008
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Setting up the NAT function
Your Gigaset SE551 WLAN dsl/cable comes provided with the NAT (Network Address
Translation) function. With Address Mapping, several users on 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 feature 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 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 particular 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. These applications cannot work if Network Address Translation (NAT)
has been activated.
Using port forwarding (the forwarding of requests to particular ports) you make the
router forward requests from the Internet for a certain service, e.g a game, to the appro-
priate 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 SE551 WLAN dsl/cable 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 approval for the 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. Then you assign to this trigger port the public ports
that have to be opened for the application.
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, 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 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.