So far we have seen how packets travel between machines on one network. Internetworking addresses transmission of packets between machines on different networks.
Communication of data between computers on different networks requires machines that connect (directly or indirectly) to both networks that are willing to shuffle packets from one network to another. Such machines are called bridges, gateways or routers. As an example, the machine ciscokid.oit.unc.edu serves as a router between our departmental FDDI network and the rest of the world. Similarly, the machine mercury.cs.unc.edu serves as a bridge between an Ethernet subnet and the main departmental FDDI backbone. Look at http://www.cs.unc.edu/compServ/network/current.html for our current network configuration.
Not every machine connected to two networks is a gateway. For instance, the machine jeffay.cs.unc.edu is connected to connected to both an ethernet and the FDDI backbone, but is not a gateway.