IP defines addresses for several important reasons. First, each device that uses TCP/IP—each TCP/IP host—needs a unique address so that it can be identified in the network. IP also defines how to group addresses together, just like the postal system groups addresses based on postal codes (like ZIP codes in the United States).
To understand the basics, examine Figure 1-9, which shows the familiar web server Larry and web browser Bob; but now, instead of ignoring the network between these two computers, part of the network infrastructure is included.
Figure 1-9 Simple TCP/IP Network: Three Routers with IP Addresses Grouped
First, note that Figure 1-9 shows some sample IP addresses. Each IP address has four numbers, separated by periods. In this case, Larry uses IP address 1.1.1.1, and Bob uses 2.2.2.2. This style of number is called a dotted-decimal notation (DDN).
Figure 1-9 also shows three groups of addresses. In this example, all IP addresses that begin with 1 must be on the upper left, as shown in shorthand in the figure as 1. . . . All addresses that begin with 2 must be on the right, as shown in shorthand as 2. . . . Finally, all IP addresses that begin with 3 must be at the bottom of the figure.
In addition, Figure 1-9 introduces icons that represent IP routers. Routers are networking devices that connect the parts of the TCP/IP network together for the purpose of routing (forwarding) IP packets to the correct destination. Routers do the equivalent of the work done by each post office site: They receive IP packets on various physical interfaces, make decisions based on the IP address included with the packet, and then physically forward the packet out some other network interface.
The TCP/IP network layer, using the IP protocol, provides a service of forwarding IP packets from one device to another. Any device with an IP address can connect to the TCP/IP network and send packets. This section shows a basic IP routing example for perspective.
Note
The term IP host refers to any device, regardless of size or power, that has an IP address and connects to any TCP/IP network.
Figure 1-10 repeats the familiar case in which web server Larry wants to send part of a web page to Bob, but now with details related to IP. On the lower left, note that server Larry has the familiar application data, HTTP header, and TCP header ready to send. In addition, the message now contains an IP header. The IP header includes a source IP address of Larry’s IP address (1.1.1.1) and a destination IP address of Bob’s IP address (2.2.2.2).
Figure 1-10 Basic Routing Example
Step 1, on the left of Figure 1-10, begins with Larry being ready to send an IP packet. Larry’s IP process chooses to send the packet to some router—a nearby router on the same LAN—with the expectation that the router will know how to forward the packet. (This logic is much like you or me sending all our letters by putting them in a nearby mailbox.) Larry doesn’t need to know anything more about the topology or the other routers.
At Step 2, Router R1 receives the IP packet, and R1’s IP process makes a decision. R1 looks at the destination address (2.2.2.2), compares that address to its known IP routes, and chooses to forward the packet to Router R2. This process of forwarding the IP packet is called IP routing (or simply routing).
At Step 3, Router R2 repeats the same kind of logic used by Router R1. R2’s IP process will compare the packet’s destination IP address (2.2.2.2) to R2’s known IP routes and make a choice to forward the packet to the right, on to Bob.
You will learn IP to more depth than any other protocol while preparing for CCNA. More than half the chapters in this book discuss some feature that relates to addressing, IP routing, and how routers perform routing.
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