This connection is handled by a dedicated host, a so-called gateway, which handles incoming and outgoing packets by copying them between the two Ethernets and the fiber optics cable. For example, if you are at the Maths Department, and want to access quark on the Physics Department's LAN from your box, the networking software cannot send packets to quark directly, because it is not on the same Ethernet. Therefore, it has to rely on the gateway to act as a forwarder. The gateway (name it sophus) then forwards these packets to its peer gateway niels at the Physics Department, using the backbone, with niels delivering it to the destination machine. Data flow between erdos and quark is shown in figure- (With apologies to Guy L. Steele).
Figure: The three steps of sending a datagram from erdos to quark.
sorry, working on it (tony :()
This scheme of directing data to a remote host is called routing, and packets are often referred to as datagrams in this context. To facilitate things, datagram exchange is governed by a single protocol that is independent of the hardware used: IP, or Internet Protocol. In chapter-, we will cover IP and the issues of routing in greater detail.
The main benefit of IP is that it turns physically dissimilar networks into one apparently homogeneous network. This is called internetworking, and the resulting ``meta-network'' is called an internet. Note the subtle difference between an internet and the Internet here. The latter is the official name of one particular global internet.
Of course, IP also requires a hardware-independent addressing scheme. This is achieved by assigning each host a unique 32-bit number, called the IP-address. An IP-address is usually written as four decimal numbers, one for each 8-bit portion, separated by dots. For example, quark might have an IP-address of 0x954C0C04, which would be written as 220.127.116.11. This format is also called dotted quad notation.
You will notice that we now have three different types of addresses: first there is the host's name, like quark, then there are IP-addresses, and finally, there are hardware addresses, like the 6-byte Ethernet address. All these somehow have to match, so that when you type rlogin quark, the networking software can be given quark's IP-address; and when IP delivers any data to the Physics Department's Ethernet, it somehow has to find out what Ethernet address corresponds to the IP-address. Which is rather confusing.
We will not go into this here, and deal with it in chapter- instead. For now, it's enough to remember that these steps of finding addresses are called hostname resolution, for mapping host names onto IP-addresses, and address resolution, for mapping the latter to hardware addresses.