Figure 2.2 shows 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 2.2: A part of the net topology at Groucho Marx University
Different physical networks have to belong to different IP networks for IP to be able to recognize if a host is on a local network. 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." sophus has been assigned the address 149.76.1.1 for use on the 149.76.1.0 network and address 149.76.4.1 for use on the 149.76.4.0 network. A gateway must be assigned one IP address for each network it belongs to. These addresses - along with the corresponding netmask - are tied to the interface through which the subnet is accessed. Thus, the interface and address mapping for sophus would look like this:
| Interface | Address | Netmask |
|---|---|---|
| eth0 | 149.76.4.1 | 255.255.255.0 |
| fddi0 | 149.76.1.1 | 255.255.255.0 |
| lo | 127.0.0.1 | 255.0.0.0 |
The last entry describes the loopback interface lo, which we talked about earlier.
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 to the host as having this-and-that IP address, although strictly speaking, it's the Ethernet interface that has this IP address. The distinction is really important only when you refer to a gateway.
The Routing Table
We now focus our attention on how IP chooses a gateway to use to deliver a datagram to a remote network.
We have seen that erdos, when given a datagram for quark, checks the destination address and finds that it is not on the local network. erdos therefore sends the datagram to the default gateway sophus, which is now faced with the same task. sophus recognizes that quark is not on any of the networks it is connected to directly, so it has to find yet another gateway to forward it through. The correct choice would be niels, the gateway to the Physics department. sophus thus needs information to associate a destination network with a suitable gateway.
IP uses a table for this task that associates networks with the gateways by which they may be reached. A catch-all entry (the default route) must generally be supplied too; this is the gateway associated with network 0.0.0.0. All destination addresses match this route, since none of the 32 bits are required to match, and therefore packets to an unknown network are sent through the default route. On sophus, the table might look like this:
| Network | Netmask | Gateway | Interface |
|---|---|---|---|
| 149.76.1.0 | 255.255.255.0 | - | fddi0 |
| 149.76.2.0 | 255.255.255.0 | 149.76.1.2 | fddi0 |
| 149.76.3.0 | 255.255.255.0 | 149.76.1.3 | fddi0 |
| 149.76.4.0 | 255.255.255.0 | - | eth0 |
| 149.76.5.0 | 255.255.255.0 | 149.76.1.5 | fddi0 |
| … | … | … | … |
| 0.0.0.0 | 0.0.0.0 | 149.76.1.2 | fddi0 |
If you need to use a route to a network that sophus is directly connected to, you don't need a gateway; the gateway column here contains a hyphen.
The process for identifying whether a particular destination address matches a route is a mathematical operation. The process is quite simple, but it requires an understanding of binary arithmetic and logic: A route matches a destination if the network address logically ANDed with the netmask precisely equals the destination address logically ANDed with the netmask.
Translation: a route matches if the number of bits of the network address specified by the netmask (starting from the left-most bit, the high order bit of byte one of the address) match that same number of bits in the destination address.
When the IP implementation is searching for the best route to a destination, it may find a number of routing entries that match the target address. For example, we know that the default route matches every destination, but datagrams destined for locally attached networks will match their local route, too. How does IP know which route to use? It is here that the netmask plays an important role. While both routes match the destination, one of the routes has a larger netmask than the other. We previously mentioned that the netmask was used to break up our address space into smaller networks. The larger a netmask is, the more specifically a target address is matched; when routing datagrams, we should always choose the route that has the largest netmask. The default route has a netmask of zero bits, and in the configuration presented above, the locally attached networks have a 24-bit netmask. If a datagram matches a locally attached network, it will be routed to the appropriate device in preference to following the default route because the local network route matches with a greater number of bits. The only datagrams that will be routed via the default route are those that don't match any other route.