CCNA RSE Lab: Configuring Port Address Translation (PAT)

CCNA RSE Lab: Configuring Port Address Translation (PAT)


Addressing Table

Part 1: Build the Network and Verify Connectivity
Part 2: Configure and Verify NAT Pool Overload
Part 3: Configure and Verify PAT

Background / Scenario
In the first part of the lab, your company is allocated the public IP address range of by the ISP. This provides the company with six public IP addresses. Dynamic NAT pool overload uses a pool of IP addresses in a many-to-many relationship. The router uses the first IP address in the pool and assigns connections using the IP address plus a unique port number. After the maximum number of translations for a single IP address have been reached on the router (platform and hardware specific), it uses the next IP address in the pool. NAT pool overload is a form port address translation (PAT) that overloads a group of public IPv4 addresses.

In Part 2, the ISP has allocated a single IP address,, to your company for use on the Internet connection from the company Gateway router to the ISP. You will use the 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.

Note: The routers used with CCNA hands-on labs are Cisco 1 941 Integrated Services Routers (ISRs) with Cisco IOS Release 1 5.2(4)M3 (universalk9 image). The switches used are Cisco Catalyst 2960s with Cisco IOS Release 1 5.0(2) (lanbasek9 image). Other routers, switches and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the commands available and output produced might vary from what is shown in the labs. Refer to the Router Interface Summary Table at the end of this lab for the correct interface identifiers.

Note: Make sure that the routers and switch have been erased and have no startup configurations. If you are unsure, contact your instructor.

Instructor Note: Refer to the Instructor Lab Manual for the procedures to initialize and reload devices.

Required Resources

  • 2 Routers (Cisco 1941 with Cisco IOS Release 15.2(4)M3 universal image or comparable)
  • 1 Switch (Cisco 2960 with Cisco IOS Release 15.0(2) lanbasek9 image or comparable)
  • 3 PCs (Windows 7, Vista, or XP with terminal emulation program, such as Tera Term)
  • Console cables to configure the Cisco IOS devices via the console ports
  • Ethernet and serial cables as shown in the topology
Part 1: Build the Network and Verify Connectivity

In Part 1 , you will set up the network topology and configure basic settings, such as the interface IP addresses, static routing, device access, and passwords.
Step 1: Cable the network as shown in the topology.
Step 2: Configure PC hosts.
Step 3: Initialize and reload the routers and switches.
Step 4: Configure basic settings for each router.
a. Console into the router and enter global configuration mode.
b. Copy the following basic configuration and paste it to the running-configuration on the router.

c. Configure the host name as shown in the topology.
d. Copy the running configuration to the startup configuration.

Step 5: Configure static routing.
a. Create a static route from the ISP router to the Gateway router.

b. Create a default route from the Gateway router to the ISP router.

Step 6: Verify network connectivity.
a. From the PC hosts, ping the G0/1 interface on the Gateway router. Troubleshoot if the pings are unsuccessful.
b. Verify that the static routes are configured correctly on both routers.

Part 2: Configure and Verify NAT Pool Overload

In Part 2, you will configure the Gateway router to translate the IP addresses from the network to one of the six usable addresses in the range.

Step 1: Define an access control list that matches the LAN private IP addresses.
ACL 1 is used to allow the network to be translated.

Step 2: Define the pool of usable public IP addresses.

Step 3: Define the NAT from the inside source list to the outside pool.

Step 4: Specify the interfaces.
Issue the ip nat inside and ip nat outside commands to the interfaces.

Step 5: Verify the NAT pool overload configuration.
a. From each PC host, ping the address on the ISP router.

b. Display NAT statistics on the Gateway router.

c. Display NATs on the Gateway router.

Note: Depending on how much time has elapsed since you performed the pings from each PC, you may not see all three translations. ICMP translations have a short timeout value.

How many Inside local IP addresses are listed in the sample output above? __________ 3
How many Inside global IP addresses are listed? __________ 1
How many port numbers are paired with the Inside global addresses? _________ 3

What would be the result of pinging the Inside local address of PC-A from the ISP router? Why?
The ping would fail because the router knows the location of the Inside global address in its routing table but the Inside local address is not advertised.

Part 3: Configure and Verify PAT

In Part 3, you will configure PAT by using an interface instead of a pool of addresses to define the outside address. Not all of the commands in Part 2 will be reused in Part 3.

Step 1: Clear NATs and statistics on the Gateway router.
Step 2: Verify the configuration for NAT.
a. Verify that statistics have been cleared.
b. Verify that the outside and inside interfaces are configured for NATs.
c. Verify that the ACL is still configured for NATs.

What command did you use to confirm the results from steps a to c?
Gateway# show ip nat statistics

Step 3: Remove the pool of useable public IP addresses.
Gateway(config)# no ip nat pool public_access 209.165.200. 225 209.165.200. 230
netmask 255.255.255. 248

Step 4: Remove the NAT translation from inside source list to outside pool.
Gateway(config)# no ip nat inside source list 1 pool public_access overload

Step 5: Associate the source list with the outside interface.
Gateway(config)# ip nat inside source list 1 interface serial 0/0/1 overload

Step 6: Test the PAT configuration.
a. From each PC, ping the address on the ISP router.
b. Display NAT statistics on the Gateway router.

c. Display NAT translations on Gateway.

What advantages does PAT provide?
Answers will vary, but should include that PAT minimizes the number of public addresses needed to provide Internet access, and that PAT, like NAT, serves to “hide” private addresses from outside networks.

Router Interface Summary Table

Router Interface Summary

Router Model Ethernet Interface #1 Ethernet Interface #2 Serial Interface #1 Serial Interface #2
1800 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
1900 Gigabit Ethernet 0/0
Gigabit Ethernet 0/1
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2801 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/1 /0 (S0/1 /0) Serial 0/1 /1 (S0/1 /1)
2811 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2900 Gigabit Ethernet 0/0 (G0/0) Gigabit Ethernet 0/1 (G0/1) Serial 0/0/0(S0/0/0) Serial 0/0/1 (S0/0/1)
Note: To find out how the router is configured, look at the interfaces to identify the type of router and how many interfaces the router has. There is no way to effectively list all the combinations of configurations for each router class. This table includes identifiers for the possible combinations of Ethernet and Serial interfaces in the device. The table does not include any other type of interface, even though a specific router may contain one. An example of this might be an ISDN BRI interface. The string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands to represent the interface.

Device Configs
Router Gateway (After Part 2)

Router Gateway (After Part 3)

Router ISP

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