INNACOMM W3100V, User Manual

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3.4.3 NAT
3.4.3.1 Overview
Setting up the NAT Function
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The DSL router is equipped with the network address translation (NAT)
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 public IP address of the router by default.
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One of the characteristics of NAT is that data from the Internet is not allowed
into the local network unless it is 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).
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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.
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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.