Network and Routing Basics B-1
Appendix BNetwork and Routing Basics
This chapterprovides an overview of IP networks, routing, and firewalls.

Basic Router Concepts

Largeamounts of bandwidth can be provided easily and relatively inexpensively in a local area
network(LAN). However, providing high bandwidth between a local network and the Internet can
be very expensive.Because of this expense, Internet access is usually provided by a slower-speed
wide-areanetwork (WAN) link such as a cable or DSL modem. In order to make the best useof the
slower WANlink, a mechanis mm ust be in place for selecting and transmitting only the data tr affic
meant forthe Internet. The function of selecting and forwarding this data is performedby a router.

What is a Router?

A router is a devicethat forwards traffic between networks based on network layer information in
thedata and on routing tables maintained by the router. In these routing tables, a routerbuilds up a
logicalpicture of the overall network by gathering and exchanging information with other routers
in the network.Using this information, the router chooses the best path for forwardingnetwork
traffic.