94
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 to pass through. The
router opens one specific port for the application. A port in
this context 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 blocks 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 (that is. several ports), so that 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 run if NAT is 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 DSL router
forwards the data from the 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 the network, but
rather to the port numbers of the required Internet service.
Where configuration is concerned, you must 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 are to be opened for the application to
this trigger port. The router checks all outgoing data for
