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Networks, Routing, and Firewall Basics B-1
Appendix B
Networks, Routing, and Firewall Basics
This chapter provides an overview of IP networks, routing, and firewalls.
Basic Router Concepts
Large amounts 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-area network (WAN) link such as a cable or DSL modem. In order to make the best use of the
slower WAN link, a mechanism must be in place for selecting and transmitting only the data traffic
meant for the Internet. The function of selecting and forwarding this data is performed by a router.
What is a Router?
A router is a device that forwards traffic between networks based on network layer information in
the data and on routing tables maintained by the router. In these routing tables, a router builds up a
logical picture 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 forwarding network
traffic.