Most of the explanations that I have provided in the other topics of this section have discussed the mechanisms by which machines connect to each other over a network directly. However, one of the most powerful aspects of networking is that it is possible to create internetworks—networks of networks—which allow devices to be connected indirectly. For example, machine “A” may send a message to machine “B” without really even knowing where it is on the network at all.

If a message is being sent between devices that are not on the same network, then it must be passed between directly-connected networks until it reaches its final destination. The process of transmitting a message from one network to another is called forwarding, and the collective process of forwarding from one device to another is routing. These concepts are fundamental to all internetworking, including the Internet itself. Every time you access an Internet resource such as a Web site, you are sending messages that get routed to that site, and the responses you receive get routed back.

In the context of the OSI Reference Model, routing is an activity that generally takes place at the network layer—layer 3. Recall that data encapsulation causes a higher-layer message to be surrounded by headers and/or footers at the lower layers. When a message is routed, here's what happens:

• A high-level application on a machine decides to send a datagram to a distant computer. The datagram is packaged, and then passed down vertically through the protocol stack on the originating machine. Each layer encapsulates the data as described earlier. The datagram is addressed to the final destination device. When the message gets to the network layer and below, however, it is not packaged for local delivery directly to its ultimate destination, but rather to an intermediate device. This is the device that is responsible for routing to that destination network. The message is passed down to the data link layer and then the physical layer for transmission to that intermediate device.
  
  
• The intermediate device (often called a router) receives the message at the physical layer. It is passed up to the data link layer, where it is processed, checked for errors and so on, and the data link layer headers are removed. The resulting packet is passed up to the network layer. There, the intermediate device determines if the destination machine is on its local network, or if it needs to be forwarded to another intermediate device. It then repackages the message and passes it back down to the data link layer to be sent on the next leg of its journey.
  
  
• After several potential intermediate devices “handle” the message, it eventually reaches its destination. Here, it travels back up the protocol stack until it reaches the same layer as the one of the application that generated the message on the originating machine.
  

  Figure 17: Message Routing in the OSI Reference Model

  This diagram shows how routing is accomplished conceptually in the OSI model. The intermediate device connects the networks of the message transmitter and recipient. When data is sent, it is passed up to the network layer on the intermediate device, where it is repackaged and sent back down the stack for the next leg of its transmission. Note that the intermediate device actually has two different layer 1 and 2 implementations; one for the interface to each network. Also note that while the layer 3 protocol must be the same across the internetwork, each network can use different technologies at layers 1 and 2.

   

  
  

The key to this description is that in the intermediate devices, the message travels back up the OSI layers only to the network layer. It is then repackaged and sent back along its way. The higher layers are only involved on the source and destination devices. The protocol used at layer 3 must be common across the internetwork but each individual network can be different. This demonstrates some of the power of layering, by enabling even rather dissimilar physical networks to be connected together. The process is illustrated in Figure 17.