In a nutshell, routers do exactly what their name says: They route data from a LAN to another router, then another router, and so on until data is received at its destination. Routers also act as traffic cops, allowing only authorized machines to transmit data into the local network so that private information can remain secure. In addition to supporting these dial-in and leased connections, routers also handle errors, keep network usage statistics, and handle security issues.
Routing for Efficiency
When you send an e-mail to your Aunt Sadie on the other side of the country, it’s routing technology that ensures she and she alone gets the message, and not every computer hooked up to the Internet. Routers direct the flow of traffic among, rather than within, networks. For instance, let’s consider how routers can be used within a LAN to keep information flowing.
Design-O-Rama, as shown in Figure 4-2, is a computer graphics company. The company’s LAN is divvied into two smaller LANs — one for the animators and one for the administration and support staff. The two subdivisions are connected with a router. Design-O-Rama employs eight people — four animators and four other staffers. When one animator sends a file to another, the large file will use a great deal of the network’s capacity. This results in performance problems for the others on the network.
NOTE: Remember how Ethernet works. A single user can have such a dramatic impact on the network because each information packet sent by one computer is broadcast to all the other computers on the LAN. Then each computer examines the packet and decides if it was meant for them.
To keep the animators from constantly slowing down the network, the network was divided into two—one for the animators and one for everybody else. A router links the two networks and connects them both to the Internet. The router is the only device on the network that sees every message sent by any computer on either network. When an animator sends a file to a colleague, the router looks at the recipient’s address and keeps that piece of traffic isolated on that LAN. On the other hand, if the animator wants to query the human relations department about vacation time, the router knows to let that piece of traffic through to the HR department.
Routers and the Internet
In our previous example, we examined how a router could be used locally. Now, let’s broaden the scope of what routers do to include their functionality across the entire Internet.
For the sake of comparison, let’s first talk about how a telephone call is routed across the country. Say it’s Aunt Sadie’s birthday and rather than send an e-mail, you want to call her. When you make a long-distance call, the telephone system establishes a stable circuit between your telephone and Aunt Sadie’s. The circuit may involve hopping through a number of steps, including fiber-optics, copper wires, and satellites. This end-to-end chain ensures that the quality of the line between you and Aunt Sadie will be constant. However, if the satellite goes offline or work crews cut the fiber-optic cable, your conversation with Aunt Sadie will be cut short. The Internet avoids this problem by making its “calls” in an entirely different way.
This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today. Buy this book now.
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