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Lesson 2
Planning an IP Routing Solution
2-17
design calls for different data-link layer protocols or different network
media to satisfy
the requirements of different users, you can connect those different
networks using
routers. You can connect two different types of Ethernet, such as connecting
a
100Base-TX Fast Ethernet horizontal LAN (using Category 5 unshielded twisted
pair
cable) to a 1000Base-SX Gigabit Ethernet backbone (using fiber-optic cable),
or even
connecting an Ethernet LAN to a Token Ring LAN.
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Data-link
Data-link
Data-link
Physical
Physical
Physical
F02pm03
Figure 2-3
A router processing network traffic
Creating WANs
In addition to connecting LANs, routers can also connect a LAN to a WAN
connection,
enabling you to join networks at different locations. This is the most
common applica-
tion for routers today. Every network connected to the Internet uses a
router to connect
the private network to an ISP’s network. The ISP in turn has its own routers
that pro-
vide the connection to the Internet. Even a simple Windows computer using
the Inter-
net Connection Sharing (ICS) feature is functioning as a router.
Some network installations also use routers and WAN connections to join
distant
offices. For example, a branch office might be connected to corporate
headquarters
using a T-1 line, which is a permanent, digital telephone connection between
the two
sites. To connect the networks at those sites, each one has a router
connecting it to one
end of the T-1, as shown in Figure 2-4. The T-1 itself then becomes a
two-node net-
work, connecting the two remote LANs. A computer at one site that has to
send traffic
to a computer at the other site sends its packets to the router on the local
network. The
router then forwards the packets over the T-1 to the router at the other
site. The second
router then forwards the packets to the LAN in the other office.
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