Gateways

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:

	----------------------------------------
	+-------+-------------+----------------+
	|iface  |    address  |       netmask  |
	+-------+-------------+----------------+
	+-------+-------------+----------------+
	|eth0   | 149.76.4.1  | 255.255.255.0  |
	|fddi0  | 149.76.1.4  | 255.255.255.0  |
	|lo     |  127.0.0.1  |     255.0.0.0  |
	+-------+-------------+----------------+
	+-------+-------------+----------------+

	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.