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Networks, Routing, and Firewall Basics

B-1

This appendix provides an overview of IP networks, routing, and firewalls.

As you read this document, you may be directed to various RFC documents for further 
information. An RFC is a Request For Comment (RFC) published by the Internet Engineering 
Task Force (IETF), an open organization that defines the architecture and operation of the Internet. 
The RFC documents outline and define the standard protocols and procedures for the Internet. The 
documents are listed on the World Wide Web at 

 and are mirrored and indexed at 

many other sites worldwide.

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.

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.

Routers vary in performance and scale, number of routing protocols supported, and types of 
physical WAN connection they support.