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Introduction

There is no Internet without the Internet Protocol (IP). IP sets the rules for communicating among Internet connected devices and serves as the foundation for every Internet app. IP is not only the protocol for the Internet but is the de facto enterprise network protocol as well. This chapter introduces the basics of IP networking and motivations for managing IP addresses within your own network.

IP Networking Overview

Each party engaged in a communication, whether two people speaking or two computers exchanging information, must comply with a set of conventions that govern the rules of such communication. Language and culture generally guide such conventions for human conversation. A protocol defines these conventions for computers. And it's usually easier to get computers to comply with these conventions than people! A protocol dictates the sequence and syntax of communications as well as recovery mechanisms required in response to error conditions. There are actually several protocols or protocol layers that facilitate computer communications, each providing a specific set of functions to support a level of commonality for communicating over a variety of media. We'll delve more into this later in this chapter, but let's start out with a simple analogy to human communications to introduce the key aspects of IP addressing and why address management is important for those who manage IP networks.

When two people converse, one person may initiate the discussion in one of many ways: by physically approaching the other and speaking, calling him or her on their mobile, sending him or her an instant message, and so on. In each of these scenarios, the initiator of the conversation identifies and locates the intended recipient, then attempts to begin a conversation using the chosen medium. When I want to talk to my friend Steve for example, I can look up his number on-line or in a phone book, dial his number, and when he answers the phone, I can identify myself and begin the conversation. At a basic level, IP communications follows a similar process. When an IP device seeks to communicate with another, it must identify and locate the intended recipient, then initiate communications over a link, while also identifying itself to the recipient in the process.

Perhaps the best, though admittedly trite, analogy for IP communications is that of postal letter delivery. Nevertheless, let's consider this process of "sneaker mail," then relate it to IP communications. The basic postal delivery process is depicted in Figure 1.1, beginning with me writing a letter to Steve and communicating it via postal mail.

After writing my letter, I enclose it in an envelope. This is step 1. Next, I write my return (From) address and Steve's (To) address on the envelope, and stamp it to pay my postal service provider. At this point, I'm ready to mail it, so step 3 consists of depositing my letter in my outgoing mailbox. After my mailperson picks up my letter, the fourth step entails forwarding of the letter within the postal system to the local post office serving Steve's address. After the letter has been delivered to the post office or distribution center serving Steve's address, a local delivery mailperson drops the letter in Steve's mailbox. When Steve walks out to the mailbox, he can open the letter and read my letter. Message delivered!

Let's map this postal message flow to sending a message over an IP network, referring to Figure 1.2. In this case, we're communicating electronically over the Internet, though this analogy holds whether communication ensues over a private enterprise, broadband, wireless, or home IP network or a combination thereof. Just as Steve and I have postal mail addresses, we both need IP addresses to communicate with each other over the Internet. No one else in the world has the same mailing address as Steve; likewise, no one else in the world has the same IP address as Steve (technically this isn't necessarily true when IP addresses are translated between me and Steve, but let's go with it for now). Let's assume that each of our computers is configured with its respective IP address and that I know Steve's IP address.

Figure 1.1 The postal delivery analogy.

Figure 1.2 Internet protocol communications.

Step 1 entails the creation or typing of my message to Steve. In step 2, my computer, knowing my IP address and Steve's, places my message within a data packet, or specifically an IP packet. An IP packet is simply the message to be communicated (including upper layer headers), prefixed with an IP header. The IP header, like our letter envelope, contains my (From) source IP address as well as Steve's (To) destination IP address, among other fields. Having formulated my IP packet, I'm now ready to send it. From my home network, I have a broadband router, to which my computer transmits my IP packet as step 3. This transmission may occur over a cable or a wireless connection between my computer and the router.

In step 4, my router forwards my IP packet to the Internet via my broadband service provider (no stamp required, they'll bill me later). Devices in the Internet called routers forward my IP packet ultimately to Steve's broadband service provider and the broadband router in his house. Routers examine each IP packet's header information to determine where to forward the packet to reach its destination IP address efficiently. Having been delivered to Steve's broadband router, step 5 consists of forwarding the packet to Steve's computer, whose IP address matches the IP packet's destination IP address field. In step 6, Steve's computer strips off the IP header to yield the message I had typed. Message delivered!

In both postal and IP communications, the source and destination addresses are specified and are unique, an infrastructure of people and/or machines successively forward the message toward its addressed destination, and it is ultimately delivered to the recipient. The table below summarizes the key similarities among the postal and IP communications examples.

The two core concepts common to these communications analogies are routing and addressing. As we've implied so far, routing is dependent on proper addressing! Let's examine this relationship in more detail.

IP Routing

The postal system operates by "routing" letters and packages as efficiently as possible to regional distribution centers, local centers, and finally to the curb. Scanning and tracking systems along the way direct parcels closer to their ultimate destination via various means of transportation through one or more distribution centers along the way. Typically, this routing is performed by first examining the "To" (destination) addressed country, postal code, city and state or province, and finally the street address. The encoding of the general (country, postal code) and the specific (street address) in the "To" address enables different entities in the postal system to use different portions of the address to route efficiently. Distribution centers can forward packages based only on country and postal code information; once the parcel arrives at a local center serving the destination postal code, the local center then needs to examine the street address for final delivery.

If Steve lives down the street, my letter will simply traverse my local post office, perhaps a distribution center and back to Steve's local post office for delivery. If Steve lives across the country, my letter will likely route from my local post office through one or more regional centers, then to Steve's local distribution center for delivery. If Steve lives in a different country, my letter will likely be required to enter the country through a customs agent. The customs agent may analyze the...