Siemens SE105 - Manual (Page 77)

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Configuration with Advanced Setup
Gigaset Router / de / A31008-E105-B100-2-19 / configure_router.fm / 05.06.2003
Configuring Special Applications
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 emails, 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. 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, network conferences and voice over the Internet,
require several links, i.e. several ports, so that the users 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. If you want to use such
applications nevertheless, then you will have to configure them as Special
Applications. This means:
◆ You define a so-called trigger port for the application and assign it the public ports
that have to be opened for the application.
◆ The router checks all outgoing data for the port number. If it recognises a match
with 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.
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You will find a detailed example of how to configure special applications
in "Practical Tips and Configuration Examples" on the supplied CD.