Lab 11.3.1: Configuring Static NAT with ISDN

Objectives:

  • To configure an ISDN connection between our Office router and the ISP's router, and configure static Network Address Translation on the Office router.

Scenario:

Our small company has recently been assigned the network number 206.222.222.0 by ARIN. Previously the company was using a private IP address on the internal network. Now that the company is going to be connected to the Internet, we need to either go to each and every workstation and configure it with a new IP address, or to use some kind of IP address translation at the router. For security reasons our company does not want the computers to be directly on the Internet, using real IP addresses. Since our company only has a limited number of computers, we have decided to use static network address translation. It is our job to configure the ISDN connection and implement the address translation.

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. Preferably IOS version 12.0.5T or later.
  3. The next task is to set up the routers and ISDN connection as in previous labs:
  • Configure IGRP routing with the AS# 123. Ensure that no routing updates will be sent out of interface BRI 0.
  • Configure the enable password to be cisco.
  • Configure the correct IP addresses on each of the routers.
  • Configure the correct ISDN switch type. (National ISDN-1)
  • Configure the correct usernames and passwords of the router to be dialed.
  • Configure the dialer list to permit IP traffic.
  • Set up the correct static or default routes.
  • Set up the correct SPIDs on each of the routers.
  • Make sure that the encapsulation type is PPP.
  • For security reasons, our Internet service provider has requested that we configure the authentication type to CHAP.
  • Configure the correct timeout values, and dialer map addresses.

Don't forget the no shutdown command on the interfaces.

  1. 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.
  2. The first step in configuring static network address translation is to establish a static translation between one of the inside local addresses (internal private network number) and one of the inside global addresses (one of our real IP addresses). Enter the global configuration prompt on the Office router and enter the following commands:

    Office(config)#ip nat inside source static 10.30.30.2 206.222.222.2
    Office(config)#ip nat inside source static 10.30.30.3 206.222.222.3
    Office(config)#ip nat inside source static 10.30.30.4 206.222.222.4

    If we needed a static translation for workstation 10.30.30.5, how would we enter the configuration information into the router?


  3. Next we need to specify which interface is on the inside of the network. NAT needs to know this information so that it can translate addresses correctly. For our Office router, this will be the ethernet 0 or fast ethernet 0/0 port. Use the following commands for this configuration.

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

  4. Now we need to specify which interface is on the outside of our network. Generally it will be the interface that is connected to the Internet. For our router it is the interface connected to the ISP router, or the BRI 0 interface.

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

  5. Now save your new configuration, and retest your connection to insure that you are still communicating across the ISDN line.
  6. Finally, we need to verify that NAT is operating correctly on our Office router. Enter the following commands and answer the questions associated with each command.

    Office#show ip nat translations

    What information did the router respond with?

    Office#show ip nat statistics

    What information did the router respond with?

    Record the number of Hits and the number of Misses

    Office#show ip nat translations verbose

    What information did the router respond with?

    Record Create and the Use

  7. From a workstation on the inside network ping an address on the outside. For us, ping the workstation on the network off of the ISP router.

    Were you successful?

    Office#show ip nat translations verbose

    What additional information did the router respond with?

    Record Create and the Use

    Office#show ip nat statistics

    What information did the router respond with?

    Record the number of Hits and the number of Misses

  8. From the ISP router, ping 206.222.222.2 (which is a statically assigned global address for the workstation 10.30.30.2 on our internal network)

    Were you successful?

    Explain why you got that result. (Hint: Check the ISP's routing table)

  9. Since we are on a stub network and the internal IP addresses are hidden from the rest of the Internet, we need to add a static route to that network on the ISP router. Now add a static route to the network 206.222.222.0/24 to the ISP router.

    What is the command to enter a static route?


  10. Try to ping the IP address 206.222.222.2 again from the ISP router.

Were you successful this time?

Why did our stub Office router not share information about network 206.222.222.0 with the ISP router?

What would happen if we incorrectly applied the inside and outside interfaces?