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, switches, gateways or routers. These denote two kinds of "internetworking" - one performed by the hardware and the other by the IP layer. Bridges or switches do hardware-supported internetworking, allowing two physical networks of the same kind (e.g. ethernet) to become one network. They send packets from one network to the other. Bridge is the traditional term for this concept while switch is the more fashionable one.
Gateways or routers do software-supported internetworking, allowing arbitrary heterogeneous networks to form one logical network. Gateway is the traditional term and router the modern one. We will focus here on gateways/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.
is connected to connected to both an ethernet and the
but is not a gateway.