Lab 11.3.5: Configuring Dynamic Overlapping NAT with ISDN

Objectives:

  • To configure an ISDN connection between Bread company router and the Butter company router, and configure it so that the two networks which have the same IP addresses can communicate together. To accomplish the communication we will configure dynamic overlapping network address translation.

Scenario:

The Bread company has recently merged with the Butter company. It is our job to connect the two companies together via an ISDN link. Our only problem is that both of the companies have implemented the same private addressing scheme on their networks. Since changing all of the IP addresses in one or both of the companies is not an option due to the large number of users at each company, we are left with only one alternative, Overlapping Network Address Translation. It is our job to connect the two companies together via ISDN and configure the appropriate address translation so that they can communicate together.

Notes: 

Lab Tasks:

  1. Cable the lab as shown in the diagram.
  2. Check to make sure that the correct Cisco IOS is loaded on the Office router. It should support network address translation.
  3. The next task is to set up the routers and ISDN connection as in previous labs:
  4. Before configuring network address translation on the Office router, test your configurations by pinging the other router. This way you are insuring that your basic configuration is correct before changing it.
  5. Luckily we only have to configure one of the routers with dynamic overlapping network address translation. There are some key terms to understand before starting to program the router. Inside local addresses are the private addresses used on the internal network. Inside global addresses the addresses that our local IP addresses are translated into on the way out of the router. Usually the Inside global addresses are real IP addresses on the Internet. Outside global addresses are the IP addresses that get translated on the way into the local network. The outside local addresses are what the outside global addresses get translated into, and are how the local network sees the remote network.

    We will be configuring the Butter router with Overlapping NAT. So for our exercise, the local area network on the Butter router (inside local addresses) sees the Bread routers LAN as network 200.245.32.0 (outside local address). But the Bread routers LAN (outside global) sees the Butter routers LAN as network 206.128.71.0 (outside local). The outside global address is the 10.30.30.0 network on the Bread router. And the inside local address is the 10.30.30.0 network on the Butter router.
  6. The first step in configuring overlapping NAT on the Butter router is to define a pool of inside global addresses to be allocated as needed. And define a pool of outside local addresses to be allocated as needed. From the Butter routers global configuration enter the following:

    Butter(config)#ip nat pool net-200 200.245.32.1 200.245.32.254 netmask 255.255.255.0

    Butter(config)#ip nat pool net-206 206.128.71.1 206.128.71.254 netmask 255.255.255.0

    Why do we need two IP address pools for NAT?


  7. Now we need to define a standard access list permitting those addresses that are to be translated.

    Butter(config)#access-list 4 permit 10.30.30.0 0.0.0.255
  8. Next we need to establish dynamic outside source translation, specifying the access list defined in the prior step. And establish dynamic inside source translation, specifying the access list defined in the previous step. This sets up the translation of internal private addresses into external addresses and the translation of external private addresses into internal addresses.

    Butter(config)#ip nat outside source list 4 pool net-206
    Butter(config)#ip nat inside source list 4 pool net-200

    Why are we using the same source list for both the inside NAT translation and the outside NAT translation?

    What would happen if we mistakenly reversed the two IP address pools and applied net-200 to the outside and net-206 to the inside?


  9. The final part of the configuration is to tell the router which interface is the inside interface and which is the outside interface. The inside interface is the one that connects to our internal network and the outside interface is the one that connects to the Bread router.

    Butter(config)#interface bri 0
    Butter(config-if)#ip nat outside

    Butter(config)#interface ethernet 0
    Butter(config-if)#ip nat inside

  10. Finally it is time to verify that NAT is operating correctly on our Office router.

    Butter#show ip nat translations

    What information did the router respond with?

    Butter#show ip nat statistics

    What information did the router respond with?


  11. Now for testing NAT. Don't forget to add a static route back to your global network on the Bread router. From a workstation on the Butter network, ping 10.30.30.2.

    Were you successful?
  12. From the Butter router enter the following:

    Butter#show ip nat translations

    What information did the router respond with?

    Butter#show ip nat translations verbose

    What information did the router respond with?

    Butter#show ip nat statistics

    What information did the router respond with?


  13. Now from the workstation on the Bread routers network. Ping the global IP address for the workstation listed in the nat translation table on the Butter router.

Were you successful?

From the same workstation ping an inside global address that is not in the nat table.

Were you successful?

Butter#show ip nat translations

Explain why you were not successful with your ping?

What is meant by "outside" as it pertains to overlapping translation?

What is meant by "local" as it pertains to overlapping translation?