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Network Address Translation

Chapter Description

Network address translation (NAT) is a function by which IP addresses within a packet are replaced with different IP addresses. This function is most commonly performed by either routers or firewalls. This sample chapter from Cisco Press focuses on NAT within routers.

Configuration Exercises

Refer to Figure 4-28 for Configuration Exercises 1–5.

Figure 4-28 The Internetwork for Configuration Exercises 1–5

1. ISP1 in Figure 4-28 has assigned the address block to AS 3. ISP2 has assigned the address block to AS 3. RTR1 and RTR2 are accepting full BGP routes from the ISP routers but do not transmit any routes to the ISPs. They run IBGP between them and OSPF on all Ethernet interfaces. No routes are redistributed between BGP and OSPF. The addresses of the router interfaces are as follows:

RTR1, E0:
RTR1, E1:
RTR1, S0:
RTR2, E0:
RTR2, E1:
RTR2, S0:

SVR1 is the DNS server authoritative for AS 3; its address is DNS1 reaches SVR1 at, whereas DNS2 reaches the same server at Write routing and NAT configurations for RTR1 and RTR2, translating inside addresses appropriately for each ISP's assigned address block. Any inside device must be able to reach either ISP, but no packets can leave AS 3 with a private source address under any circumstance.

2. The address of SVR2 in Figure 4-28 is, and the address of SVR3 is Modify the configurations of Configuration Exercise 1 so that devices within ISP1's AS connect to the servers round-robin at the address

3. HTTP packets sent to from ISP2 are sent to SVR2 in Figure 4-28. SMTP packets sent to from ISP2 are sent to SVR3. Modify the configurations of the previous exercises to implement these translations.

4. Five outside devices in Figure 4-28,–, must appear to devices within AS 3 as having addresses–, respectively. Add the appropriate NAT configurations to the previously created configurations.

5. Devices in AS 3 of Figure 4-28 with addresses in the subnet should all appear to have the IG address when sending packets to ISP2. Modify the configurations of the previous exercises to accommodate this.

6. In Figure 4-29, redundant links have been added so that RTR1 and RTR2 each have connections to both ISPs, and each accept full BGP routes from both ISPs. The address of RTR1, S1 is, and the address of RTR2, S1 is Write configurations for the two routers, ensuring that all features added in the previous exercises still work correctly.

Figure 4-29 The Internetwork for Configuration Exercise 6

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