Adding Routers

Recall that in TCP/IP, two types of entities exist on a network: hosts and routers. All networks must have hosts, while not all networks require routers. Whether you use routers should depend on the physical topology of the network. This section introduces the concepts of network topology and routing, important when you decide to add another network to your existing network environment.

Network Topology

Network topology describes how networks fit together. Routers are the entities that connect networks to each other. From a TCP/IP perspective, a router is any machine that has two or more network interfaces. However, the machine cannot function as a router until properly configured, as described in "Configuring Routers".

Two or more networks can be connected together by routers to form larger internetworks. The routers must be configured to pass packets between two adjacent networks. They also should be able to pass packets to networks that lie beyond the adjacent networks.

The following figure shows the basic parts of a network topology. The first illustration shows a simple configuration of two networks connected by a single router. The second shows a configuration of three networks, interconnected by two routers. In the first case, network 1 and network 2 are joined into a larger internetwork by router R. In the second case, router R1 connects networks 1 and 2, and router R2 connects networks 2 and 3, thus forming a network made up of networks 1, 2, and 3.

Figure 5-1 Basic Network Topology

Routers join networks into internetworks and route packets between them based on the addresses of the destination network. As internetworks grow more complex, each router must make more and more decisions regarding where packets are to be sent.

A step up in complexity is the case shown in the following figure. Networks 1 and 3 are directly connected by a router R3. The reason for such redundancy is reliability. If network 2 goes down, router R3 still provides a route between networks 1 and 3. Any number of networks can be interconnected and communicate as long as they all adhere to the same network protocols.

Figure 5-2 Providing an Additional Path Between Networks

How Routers Transfer Packets

Routing decisions on a network are based on the network portion of the IP address of the recipient that is contained in the packet header. If this address includes the network number of the local network, the packet goes directly to the host with that IP address. If the network number is not the local network, the packet goes to the router on the local network.

Routers maintain routing information in routing tables. These tables contain the IP address of the hosts and routers on the networks to which the router is connected. The tables also contain pointers to these networks. When a router gets a packet, it consults its routing table to see if it lists the destination address in the header. If the table does not contain the destination address, the router forwards the packet to another router listed in its routing table. Refer to "Configuring Routers" for detailed information on routers.

The following figure shows a network topology with three networks connected by two routers.

Figure 5-3 Three Interconnected Networks

Router R1 connects networks 192.9.200 and 192.9.201. Router R2 connects networks 192.9.201 and 192.9.202. If host A on network 192.9.200 sends a message to host B on network 192.9.202, this is what happens:

1. Host A sends a packet out over network 192.9.200. The packet header contains the IPv4 address of the recipient host B, 192.9.202.10.

2. None of the machines on network 192.9.200 has the IPv4 address 192.9.202.10. Therefore, router R1 accepts the packet.

3. Router R1 examines its routing tables. No machine on network 192.9.201 has the address 192.9.202.10. However, the routing tables do list router R2.

4. R1 then selects R2 as the "next hop" router and sends the packet to R2.

5. Because R2 connects network 192.9.201 to 192.9.202, it has routing information for host B. Router R2 then forwards the packet to network 192.9.202, where it is accepted by host B.