Purpose: This chapter provides an overview of Network Address Translation (NAT) and Port Address Translation (PAT). It then discusses how to enable it on a Cisco router. Timing: This chapter takes approximately 2 hours to present. Note: This section has a laboratory exercise that allows students to enable NAT on a Cisco router. Contents: Objectives—This section explains what the student will be able to do at the end of this chapter. NAT Overview—This section introduces students to NAT. NAT Operation—This section describes how NAT works. Configuring NAT—This section describes how to configure NAT on a Cisco IOS™ router. Verifying and Troubleshooting NAT—This section describes how to verify NAT configuration. Configuring and Troubleshooting PAT on the 700 Router—This section describes how to enable PAT on a 700 series Cisco router. Laboratory Exercise—Students will configure NAT. Summary—This section summarizes what was taught in the chapter. Review Questions—This section offers open-ended review questions. They should foster discussion after presenting the chapter. Transition: Following are the performance objectives that describe what students will be able to do at the end of the chapter.
Purpose: This figure states the chapter objectives. Emphasize: Read or state each objective so each student has a clear understanding of the chapter objectives.
Purpose: This figure is a graphical illustration of the chapter objectives.
Purpose: This figure describes circumstances when you would implement NAT. Emphasize: Examples when NAT may be employed include two companies that have duplicate internal addressing schemes merge, or a company changes its Internet Service Provider (ISP) but does not want to change its internal address scheme. Transition: Advantages and disadvantages of using NAT follow.
Purpose: This figure describes advantages and disadvantages of implementing NAT. Note: The most obvious advantage is that NAT conserves the legally registered address scheme. Transition: An overview of NAT follows.
Purpose: This figure is a transition into the NAT overview section. It also highlights some important NAT terms. Emphasize: Highlight the different sending addresses on the packet before it enters the router and after it leaves the router. Compare those addresses to those listed on the NAT table. Describe each term as it relates to the figure. Note: The letters on the figure correspond to the descriptions in the text. Descriptions for outside local IP address and extended translation entry are not represented graphically. Easy IP is a related feature to NAT available on Cisco routers. Configuring Easy IP is not taught in this course. The Easy IP (Phase 1) feature combines NAT and Point-to-point (PPP)/Internet Protocol Control Protocol (IPCP). This feature enables a Cisco router to automatically negotiate its own registered WAN interface IP address from a central server and enable all remote hosts to access the global Internet using this single registered IP address. Because Easy IP (Phase 1) uses existing port-level multiplexed NAT functionality within the Cisco IOS software, IP addresses on the remote LAN are invisible to the Internet. Reference: For a complete description of the Easy IP configuration commands, refer to the “Easy IP Commands” chapter in the Dial Solutions Command Reference.
Purpose: This figure is a transition that highlights the NAT functions that are presented in the next few figures. Emphasize: The next few figures discuss the following NAT functions: Translating inside global addresses Overloading inside global addresses Handling overlapping networks Transmission Control Protocol (TCP) load distribution Transition: The next figure describes translating inside global addresses.
Purpose: This figure explains how address translation works. Emphasize: Later when students learn to configure address translation, they will be able to use either static NAT configuration or dynamic NAT configuration. Transition: The next figure describes overloading inside global addresses.
Purpose: This figure explains how overloading inside global addresses works. Emphasize: Overloading inside global address translation is Port Address Translation (PAT). How to configure PAT on a Cisco 700 series router is described later in this chapter. Transition: The next figure describes TCP load distribution.
Purpose: This figure describes TCP load distribution. Emphasize: Load distribution is used when multiple inside stations have mirrored resources, requiring a unique virtual addressing scheme. Transition: The next figure describes handling overlapping networks.
Purpose: This figure describes overlapping networks. Transition: The following figures describe how to configure a Cisco IOS router to enable the NAT features just discussed.
Purpose: This figure displays the static NAT configuration output. Emphasize: Highlight the inside and outside interfaces on this configuration. Note: This figure and the subsequent NAT configuration figures only display the configurations necessary to configure NAT translation. Other commands may be necessary for routing.
Purpose: This figure displays the dynamic NAT configuration output.
Purpose: This figure describes how to configure inside global address overloading.
Purpose: This figure describes how to configure TCP load distribution.
Purpose: This figure describes how to configure NAT to translate overlapping addresses.
Purpose: This figure describes how to verify your NAT configuration output. Emphasize: The upper output box displays the typical NAT table. The lower output box displays the NAT table with overloading. Note: When looking at the IP NAT translations, you may see many translations from the same host to the same host at the destination. This is typical of many connections to the Web.
Purpose: This figure describes the debug ip nat command. Emphasize: Show the sending address, the translation, and the destination address on each debug line.
Purpose: This figure describes how to clear your NAT entries from the translation table. Emphasize: The “*” clears all entries from the NAT table. Both output boxes in the figure show how the NAT table looks before and after translations are cleared.
Purpose: Review the summary items with your students. Emphasize: Read or restate the summary statements. By now, your presentation and classroom discussion should have students able to meet the chapter learning objectives.
Purpose: Review the chapter with the open-ended questions. Note: The questions in this section are open-ended questions designed to foster further discussion. Answers to the review questions are in Appendix B, “Answers to the Review Questions.”