We will now turn to the details you'll come in touch with when connecting your machine to a TCP/IP network including dealing with IP addresses, host names, and sometimes routing issues. This chapter gives you the background you need in order to understand what your setup requires, while the next chapters will cover the tools to deal with these.
Subnetting is not only an organizational benefit, it is frequently a natural consequence of hardware boundaries. The viewpoint of a host on a given physical network, such as an Ethernet, is a very limited one: the only hosts it is able to talk to directly are those of the network it is on. All other hosts can be accessed only through so-called gateways. A gateway is a host that is connected to two or more physical networks simultaneously and is configured to switch packets between them.
For IP to be able to easily recognize if a host is on a local physical network, different physical networks have to belong to different IP networks. For example the network number 149.76.4.0 is reserved for hosts on the mathematics LAN. When sending a datagram to quark, the network software on erdos immediately sees from the IP address, 149.76.12.4, that the destination host is on a different physical network, and therefore can be reached only through a gateway (sophus by default).
sophus itself is connected to two distinct subnets: the Mathematics Department, and the campus backbone. It accesses each through a different interface, eth0 and fddi0, respectively. Now, what IP address do we assign it? Should we give it one on subnet 149.76.1.0, or on 149.76.4.0?
The answer is: both. When talking to a host on the Maths LAN, sophus should use an IP address of 149.76.4.1, and when talking to a host on the backbone, it should use 149.76.1.4.
Thus, a gateway is assigned one IP address per network it is on. These addresses --- along with the corresponding netmask --- are tied to the interface the subnet is accessed through. Thus, the mapping of interfaces and addresses for sophus would look like this:
The last entry describes the loopback interface lo, which was introduced above.
Figure #introfigip#93> shows a part of the network topology at Groucho Marx University (GMU). Hosts that are on two subnets at the same time are shown with both addresses.
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figures/groucho.epsf
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