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TCP/IP Network Layer

The application layer includes many protocols. The transport layer includes fewer protocols,
most notably, TCP and UDP. The TCP/IP network layer includes a small number
of protocols, but only one major protocol: the Internet Protocol (IP) . In fact, the name
TCP/IP is simply the names of the two most common protocols (TCP and IP) separated
by a /.

IP provides several features, most importantly, addressing and routing. This section
begins by comparing IP’s addressing and routing with another commonly known system
that uses addressing and routing: the postal service. Following that, this section introduces
IP addressing and routing. (More details follow in Chapter 4, “Fundamentals of IPv4
Addressing and Routing.”)

Internet Protocol and the Postal Service
Imagine that you just wrote two letters: one to a friend on the other side of the country
and one to a friend on the other side of town. You addressed the envelopes and put on
the stamps, so both are ready to give to the postal service. Is there much difference in
how you treat each letter? Not really. Typically, you would just put them in the same
mailbox and expect the postal service to deliver both letters.

The postal service, however, must think about each letter separately, and then make a
decision of where to send each letter so that it is delivered. For the letter sent across
town, the people in the local post office probably just need to put the letter on another

For the letter that needs to go across the country, the postal service sends the letter to
another post office, then another, and so on, until the letter gets delivered across the
country. At each post office, the postal service must process the letter and choose where
to send it next.

To make it all work, the postal service has regular routes for small trucks, large trucks,
planes, boats, and so on, to move letters between postal service sites. The service must be
able to receive and forward the letters, and it must make good decisions about where to
send each letter next, as shown in Figure 1-8.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 1-8 Postal Service Forwarding (Routing) Letters
Still thinking about the postal service, consider the difference between the person sending
the letter and the work that the postal service does. The person sending the letters expects
that the postal service will deliver the letter most of the time. However, the person sending
the letter does not need to know the details of exactly what path the letters take. In
contrast, the postal service does not create the letter, but it accepts the letter from the customer.
Then, the postal service must know the details about addresses and postal codes that
group addresses into larger groups, and it must have the ability to deliver the letters.

The TCP/IP application and transport layers act like the person sending letters through the
postal service. These upper layers work the same way regardless of whether the endpoint
host computers are on the same LAN or are separated by the entire Internet. To send a message,
these upper layers ask the layer below them, the network layer, to deliver the message.

The lower layers of the TCP/IP model act more like the postal service to deliver those messages
to the correct destinations. To do so, these lower layers must understand the underlying
physical network because they must choose how to best deliver the data from one host
to another.

So, what does this all matter to networking? Well, the network layer of the TCP/IP networking
model, primarily defined by the Internet Protocol (IP), works much like the postal
service. IP defines that each host computer should have a different IP address, just as the
postal service defines addressing that allows unique addresses for each house, apartment,
and business. Similarly, IP defines the process of routing so that devices called routers can
work like the post office, forwarding packets of data so that they are delivered to the correct
destinations. Just as the postal service created the necessary infrastructure to deliver
letters—post offices, sorting machines, trucks, planes, and personnel—the network layer
defines the details of how a network infrastructure should be created so that the network
can deliver data to all computers in the network.

NOTE TCP/IP defines two versions of IP: IP version 4 (IPv4) and IP version 6 (IPv6). The
world still mostly uses IPv4, so this introductory part of the book uses IPv4 for all references
to IP. Later in this book, Part VIII, “IP Version 6,” discusses this newer version of the
IP protocol.

Internet Protocol Addressing Basics
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.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
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, and Bob uses
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.

IP Routing Basics
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 ( and a destination IP address of Bob’s IP address (
Free CISCO CCNA Routing and Switching ICND1 Study Guide
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 (, 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 ( 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 CCENT
and CCNA. Practically half the chapters in this book discuss some feature that relates to
addressing, IP routing, and how routers perform routing.

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