Topology Diagram
Download Packet Tracer File
Objectives
• Examine NAT processes as traffic traverses a NAT border router.
Background / Preparation
In this activity, you will use Packet Tracer Simulation mode to examine the contents of the IP header as traffic crosses the NAT border router.
Step 1: Prepare the network for Simulation mode.
Verify that the network is ready to send and receive traffic. All the link lights should be green. If some link lights are still amber, you can switch between Simulation and Realtime mode several times to force the lights to turn green faster. Switch to Simulation mode before going to the next step.
Step 2: Send an HTTP request from an inside host to an outside web server.
a. Click Customer PC. Click the Desktop tab and then Web Browser. In the URL field, type the web address for the ISP server (www.ispserver.com). Make sure that you are in Simulation mode, and then click Go.
b. In the event list, notice that Customer PC queues a DNS request and sends out an ARP request. You can view the contents of the ARP request by either clicking on the packet in the topology or clicking on the packet color under Info in the Event List window.
c. In the PDU Information at Device: Customer PC window, which IP address is Customer PC attempting to find a MAC address for?
d. In the Event List window, click Capture/Forward twice. Which device answers the ARP request from Customer PC? Which MAC address is placed inside the ARP reply?
e. In the Event List window, click Capture/Forward twice. Customer PC accepts the ARP replay and then builds another packet. What is the protocol for this new packet? If you click Outbound PDU Details for this packet, you can see the details of the protocol.
f. In the Event List window, click Capture/Forward twice. Click the packet at the www.customerserver.com server. Then click the Outbound PDU Details tab. Scroll down to the bottom to see the Application Layer data. What is the IP address for the ISP server?
g. In the Event List window, click Capture/Forward twice. Customer PC now formulates another ARP request. Why?
h. In the Event List window, click Capture/Forward 10 times until Customer PC formulates an HTTP request packet. Customer PC finally has enough information to request a web page from the ISP server.
i. In the Event List window, click Capture/Forward three times. Click the packet at Customer Router to examine the contents. Customer Router is a NAT border router. What is the inside local address and the inside global address for Customer PC?
j. In the Event List window, click Capture/Forward seven times until the HTTP reply reaches Customer Router. Examine the contents of the HTTP reply and notice that the inside local and global addresses have changed again as the packet is forwarded on to Customer PC.
Step 3: Send an HTTP request from an outside host to an inside web server.
Customer Server provides web services to the public (outside addresses) through the domain name www.customerserver.com. Follow a process similar to Step 2 to observe an HTTP request on ISP Workstation.
a. Click ISP Workstation. Click the Desktop tab, and then Web Browser. In the URL field, type the Customer Server web address (www.customerserver.com). Make sure that you are in Simulation mode, and then click Go.
b. You can either click Auto Capture/Play or Capture/Forward to step through each stage of the process. The same ARP and DNS processes occur before the ISP Workstation can formulate an HTTP request.
c. When the HTTP request arrives at Customer Router, check the packet contents. What is the inside local address? What is the inside global address?
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