Introduction

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Introduction

Amazingly enough, many chapters and even whole books are dedicated to the concept of routing—discussing the types of routing, how routing works, different kinds of routers, problems encountered with routing, streamlining route tables, and so on. But very few discuss the most fundamentally important question of all: Why do you have to route in the first place? To help you fully understand routing, this chapter begins with a continuation of some of the networking concepts learned in Chapter 2. After the basics are covered, this chapter discusses the reasons for routing, and the benefits of doing so.

Recall from Chapter 2 on the architecture of networks that protocols are written to a standard networking model. Also recall that each layer of the networking model serves as an intermediary to higher layers of the model. Therefore, each layer knows how to communicate with another layer of its type, but has no idea what’s going on in layers more than one level removed, either above or below it. In the mail example, the mailman has no clue what kind of message was written, what kind of paper was used, or whether the message was written in English. The only interface between the two layers is the address on the outside of the envelope, which is all the mailman needs. Looking at the networking model then, a frame at the network interface layer would look something like Figure 5.1.

Notice in this example that the network interface layer can identify the destination hardware address, the source hardware address, the type of frame (802.3 ethernet, 802.5 token ring, and so on), and then data. The Network Interface layer has no idea what is in the data layer; it just knows that it’s supposed to send the data to the destination hardware address indicated at the front of the packet. Based on the type of communication initiated, the destination address may be all FFs or an actual unique 6-byte physical address.

All this presupposes that the voltage including this information reaches its destination. Recall from Chapter 3 that on an ethernet network each machine transmits on a network segment to communicate. The number of machines that can communicate on a network segment is limited by the machines’ capability to sense collisions and retransmit data. Networks are said to reach, or be close to, bandwidth saturation, when the machines are unable to avoid collisions while trying to communicate. The best way to avoid bandwidth saturation is to design your ethernet network so that traffic, in the form of voltage, is as segmented and isolated from other traffic as possible. Physical grouping of computers with devices such as bridges and routers minimizes the number of machines within a collision domain, or the physical part of the network that machines have to share to send and receive data.

Many network devices have been created to help in this process, to extend network segments and to isolate network traffic. To strengthen your understanding of these concepts, a review of each type of device follows. The author encourages those who are already familiar with these devices and how they work to feel free to skip these sections and move straight to the section titled “Understanding Routing.” If these devices still raise some questions in your mind, the summary is provided to fill in conceptual gaps that may exist.

As this is a chapter devoted to IP routing, an in-depth discussion of routers as devices is reserved for later in the chapter, beginning with the “Understanding Routing” section.

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