The cylindrical icons in the figure represent routers, and the rectangular icons represent
Ethernet LAN switches. Routers are networking devices that connect to multiple
physical networks, such as the multiple Ethernet networks in the figure. Routers
also forward data from one network to the next. When you drive in your car and
reach an intersection, the road signs tell you where to turn; in networking, the
routers create the equivalent of a road sign, but the router tells the data
packets which way to turn. The complete process by which a computer sends the
data, passing though all the routers and eventually arriving at the destination,
is called routing. The following list shows
the three main routing steps in Figure
10-1:
|
1. |
Hannah sends her data to R1, much like a driver might drive
to where he can get on the interstate highway. To send the data to R1, Hannah's
PC sends an Ethernet frame to R1's MAC address, much like a driver might use
local roads to reach the on-ramp to the highway.
|
|
2. |
R1 knows that to get the data to a computer in Cincinnati, it
must forward the data to R2, instead of sending the data to R3 next. This
process is much like a driver choosing which direction to go on the interstate
highway, based on the road signs. (Because the physical network between R1 and
R2 is an Ethernet, R1 forwards the data to R2 using an Ethernet
frame.)
|
|
3. |
R2 sends the data to the destination computer over the LAN
much like the driver finishes his trip by getting off the interstate highway and
driving over the local roads in Cincinnati. |
If you consider these steps a little more closely, three
devices need to forward the data at some point: Hannah, R1, and R2, in
succession. Each time, to get the data to the next device, the devices need to
send the data over an Ethernet LANand you already know how that works from
reading Chapters 4, "How to Build a
Local (Network) Roadway," through 7,
"Adding Local (Network) Roadways for No Extra Money." In sequence, Hannah uses
Ethernet to send the data to R1; R1 uses the second Ethernet network to send the
data to R2; finally, R2 uses the third Ethernet to send the data to the web
server.
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 |
As I mentioned before, the word "network" can be used in a lot
of different ways. In one way of thinking, Figure 10-1 shows a single network, but in another way of
thinking, it shows four Ethernet networks, which are then separated by three
routers. Another related term, called internetwork, is
sometimes used when you need a more exact term. If you consider each of the four
Ethernets in Figure 10-1 as individual
networks, you can think of the whole diagram as an internetwork, which is short
for "interconnected networks." Keep in mind that people use the term "network"
in a lot of different
ways. | |
Although Figure 10-1
shows only LANs, the routers could be connected to a wide-area network (WAN). In
fact, one of the main benefits of routers, besides helping forward data through
the network, is to connect to many different types of physical networks. In
later chapters, particularly in Chapters
14, "Leasing a (Network) Roadway Between Two Points," and Chapter 15, "Leasing a (Network) Roadway Between Lots
of Places," you will learn how routers use WANs to transmit data, much like they
can use LANs to transmit data. Regardless, the basic process of routing happens
the same way, whether you use LANs, WANs, or a combination of the two. In fact,
one of the reasons that routers are popular is because they can connect many
different types of physical networks with ease.
This first simple example portrays the concepts of routing,
which is the overall process by which computers and routers together forward the
data to the correct destination. To appreciate more of the details of routing,
you need to have a better understanding of IP, IP addresses, and how they affect
routing.
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