Objectives
• Configure a router to use port address translation (PAT) to convert internal IP addresses, typically
private addresses, into outside public addresses.
• Verify connectivity.
• Verify PAT statistics.
Background / Preparation
An ISP has allocated to a company a single IP address, 209.165.201.33, to be used on the Internet
connection from the company gateway router to the ISP. A static route will be used between the ISP and the
gateway router, and a default route will be used between the gateway and the ISP router. The ISP connection
to the Internet will be represented by a loopback address on the ISP router.
In this lab, you will configure the gateway router to use PAT to convert multiple internal addresses into the
one usable public address. You will test, view, and verify that the translations are taking place, and you will
interpret the NAT/PAT statistics to monitor the process.
The following resources are required:
• One Cisco 2960 switch or other comparable switch
• Two routers, each with a serial connection and one Ethernet interface to connect to the switch
• Two Windows-based PCs, one with a terminal emulation program, and both set up as hosts
• At least one RJ-45-to-DB-9 connector console cable to configure the router and switches
• Three straight-through Ethernet cables to connect from the router to Switch 1 and to connect both
hosts to the switch
• One serial cable to connect from Router 1 to Router 2
NOTE: Make sure that the routers and the switches have been erased and have no startup configurations.
Instructions for erasing both switch and router are provided in the Lab Manual, located on Academy
Connection in the Tools section.
NOTE: SDM Enabled Routers – If the startup-config is erased in an SDM enabled router, SDM will no longer
come up by default when the router is restarted. It will be necessary to build a basic router configuration using
IOS commands. The steps provided in this lab use IOS commands and do not require the use of SDM. If you
wish to use SDM, refer to the instructions in the Lab Manual, located on the Academy Connection in the Tools
section or contact your instructor if necessary.
Step 1: Connect the equipment
a. Connect Router 1 Serial 0/0/0 interface to Router 2 Serial 0/0/0 interface using a serial cable.
b. Connect Router 1 Fa0/0 interface to the Switch 1 Fa0/1 interface using a straight-through cable.
c. Connect a PC with a console cable to perform configurations on the routers and switch.
d. Connect both hosts to ports Fa0/2 and Fa0/3 on the switch using straight-through cables.
Step 2: Perform basic configurations on Router 2
a. Connect a PC to the console port of Router 2 to perform configurations using a terminal emulation
program.
b. Configure Router 2 with a hostname, interfaces, console, Telnet, and privileged passwords according
to the table diagram. Save the configuration.
Step 3: Configure the gateway router
Perform basic configuration on Router 1 as the Gateway router with a hostname, interfaces, console, Telnet,
and privileged passwords according to the table diagram. Save the configuration.
Step 4: Configure Switch 1
Configure Switch 1 with a hostname, console, Telnet, and privileged passwords according to the table
diagram.
Step 5: Configure the hosts with the proper IP address, subnet mask, and default gateway
Configure each host with the proper IP address, subnet mask, and default gateway. Both hosts should
receive IP addresses in the 10.10.10.0/24 network. The default gateway should be the FastEthernet interface
IP address of the Gateway router.
Step 6: Verify that the network is functioning
From the attached hosts, ping the FastEthernet interface of the default gateway router.
Was the ping from Host 1 successful? __________
Was the ping from Host 2 successful? __________
If the answer is no for either question, troubleshoot the router and host configurations to find the error.
Ping again until they are both successful.
Predict: If you attempted to ping the loopback IP address on ISP, would the ping be successful?
Explain your answer.
________________________________________________________________________________
________________________________________________________________________________
Step 7: Create a default route
a. From the Gateway router to the ISP router, create a static route to network 0.0.0.0 0.0.0.0, using the
ip route command. This will forward any unknown destination address traffic to the ISP by setting
a Gateway of Last Resort on the Gateway router.
Gateway(config)#ip route 0.0.0.0 0.0.0.0 209.165.201.34
b. View the routing table on the Gateway router to verify the default route.
Is the static route in the routing table? __________
c. Try to ping from one of the workstations to the ISP serial interface IP address.
Was the ping successful? __________
Why? ______________________________________________________________________
Step 8: Define the pool of usable public IP addresses
To define the pool of public addresses, use the ip nat pool command.
Gateway(config)#ip nat pool public_access 209.165.201.33 209.165.201.33
netmask 255.255.255.252
Step 9: Define an access list that will match the inside private IP addresses
To define the access list to match the inside private addresses, use the access-list command.
Gateway(config)#access-list 1 permit 10.10.10.0 0.0.0.255
Step 10: Define the NAT translation from inside list to outside pool
To define the NAT translation, use the ip nat inside source command.
Gateway(config)#ip nat inside source list 1 pool public_access overload
Step 11: Specify the interfaces
The active interfaces on the router need to be specified as either inside or outside interfaces with respect to
NAT. To do this, use the ip nat inside or ip nat outside command.
Gateway(config)#interface fastethernet 0/0
Gateway(config-if)#ip nat inside
Gateway(config-if)#interface serial 0/0/0
Gateway(config-if)#ip nat outside
Step 12: Generate traffic from Gateway to the ISP
From Host 1 PC, ping 172.16.1.1. Open multiple DOS windows on each workstation and Telnet to the
172.16.1.1 address.
Step 13: Verify that NAT/PAT is working
a. To view the NAT statistics type the show ip nat statistics command at the privileged EXEC
mode prompt on the Gateway router.
How many active translations have taken place? __________
How many addresses are in the pool? __________
How many addresses have been allocated so far? __________
b. When successful, look at the NAT translation on the Gateway router, using the command show ip
nat translations.
Gateway#show ip nat translations
Pro Inside global Inside local Outside local Outside global
icmp 209.165.201.33:2 10.10.10.10:2 172.16.1.1:2 172.16.1.1:2
icmp 209.165.201.33:3 10.10.10.10:3 172.16.1.1:3 172.16.1.1:3
icmp 209.165.201.33:4 10.10.10.10:4 172.16.1.1:4 172.16.1.1:4
icmp 209.165.201.33:5 10.10.10.10:5 172.16.1.1:5 172.16.1.1:5
icmp 209.165.201.33:6 10.10.10.10:6 172.16.1.1:6 172.16.1.1:6
How can you tell that PAT is using a single IP address for all translations? _________________
____________________________________________________________________________
What feature of the translation chart illustrates how PAT is able to keep each data translation
separate from the others? _______________________________________________________
____________________________________________________________________________
Step 14: Adjust the Gateway configuration to use an alternate PAT approach
a. Clear the NAT translation table.
Gateway#clear ip nat translation *
b. Remove the command that created a NAT pool.
Gateway(config)#no ip nat pool public_access 209.165.201.33
209.165.201.33 netmask 255.255.255.252
c. Remove the command that associated the pool with your ACL.
Gateway(config)#no ip nat inside source list 1 pool public_access
overload
d. Enter a command that associates the source list with the outside interface.
Gateway(config)#ip nat inside source list 1 interface serial 0/0/0
overload
e. Verify that this alternate approach works by generating traffic from the hosts to the loopback, and
then by using the show ip nat statistics and show ip nat translations commands.
Results should be similar to those achieved using the NAT pool.