A machine that has interfaces (adapters or serial lines) to more than one network may operate as a router (or gateway) by forwarding packets from one network to another.
When you configure a second interface on a system, you can turn it into a router or you can choose to leave it in the default non-gateway mode. If you do not make your machine into a router, it will send and receive packets on each of its interfaces that is connected to a network, but it will not forward packets that it receives on one interface to be retransmitted on another interface.
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