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.