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 188.8.131.52 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, 184.108.40.206, 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 220.127.116.11, or on 18.104.22.168?
The answer is: both. When talking to a host on the Maths LAN, sophus should use an IP address of 22.214.171.124, and when talking to a host on the backbone, it should use 126.96.36.199.
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 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.
Figure: A part of the net topology at Groucho Marx Univ.
Generally, you can ignore the subtle difference between attaching an address to a host or its interface. For hosts that are on one network only, like erdos, you would generally refer of the host as having this-and-that IP address although strictly speaking, it's the Ethernet interface that has this IP address. However, this distinction is only really important when you refer to a gateway.