CCNA RSE Lab: 9.2.2.6 Configuring Dynamic and Static NAT

CCNA RSE Lab: 9.2.2.6 Configuring Dynamic and Static NAT

Topology

Addressing Table

Objectives
Part 1: Build the Network and Verify Connectivity
Part 2: Configure and Verify Static NAT
Part 3: Configure and Verify Dynamic NAT

Background / Scenario
Network Address Translation (NAT) is the process where a network device, such as a Cisco router, assigns a public address to host devices inside a private network. The main reason to use NAT is to reduce the number of public IP addresses that an organization uses because the number of available IPv4 public addresses is limited.

In this lab, an ISP has allocated the public IP address space of 209.165.200.224/27 to a company. This provides the company with 30 public IP addresses. The addresses, 209.165.200.225 to 209.165.200.241 , are for static allocation and 209.165.200.242 to 209.165.200.254 are for dynamic allocation. A static route is used from the ISP to the gateway router, and a default route is used from the gateway to the ISP router. The ISP connection to the Internet is simulated by a loopback address on the ISP router.

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)
  • 2 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.
Attach the devices as shown in the topology diagram, and cable as necessary.

Step 2: Configure PC hosts.
Step 3: Initialize and reload the routers and switches as necessary.
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: Create a simulated web server on ISP.
a. Create a local user named webuser with an encrypted password of webpass.

b. Enable the HTTP server service on ISP.

c. Configure the HTTP service to use the local user database.

Step 6: Configure static routing.
a. Create a static route from the ISP router to the Gateway router using the assigned public network address range 209.165.200.224/27.

b. Create a default route from the Gateway router to the ISP router. Gateway(config)# ip route 0.0.0.0 0.0.0.0 209.165.201.17

Step 7: Save the running configuration to the startup configuration.
Step 8: Verify network connectivity.
a. From the PC hosts, ping the G0/1 interface on the Gateway router. Troubleshoot if the pings are unsuccessful.
b. Display the routing tables on both routers to verify that the static routes are in the routing table and configured correctly on both routers.

Part 2: Configure and Verify Static NAT

Static NAT uses a one-to-one mapping of local and global addresses, and these mappings remain constant. Static NAT is particularly useful for web servers or devices that must have static addresses that are accessible from the Internet.

Step 1: Configure a static mapping.
A static map is configured to tell the router to translate between the private inside server address 192.168.1.20 and the public address 209.165.200.225. This allows a user from the Internet to access PC-A. PC-A is simulating a server or device with a constant address that can be accessed from the Internet.

Step 2: Specify the interfaces.
Issue the ip nat inside and ip nat outside commands to the interfaces.

Step 3: Test the configuration.
a. Display the static NAT table by issuing the show ip nat translations command.

What is the translation of the Inside local host address?
192.168.1.20 = _________________________________________________________ 209.165.200.225

The Inside global address is assigned by?
The router from the NAT pool.

The Inside local address is assigned by?
The administrator for the workstation.

b. From PC-A, ping the Lo0 interface (192.31.7.1 ) on ISP. If the ping was unsuccessful, troubleshoot and correct the issues. On the Gateway router, display the NAT table.

A NAT entry was added to the table with ICMP listed as the protocol when PC-A sent an ICMP request (ping) to 192.31.7.1 on ISP.

What port number was used in this ICMP exchange? ________________ 1, answers will vary.

Note: It may be necessary to disable the PC-A firewall for the ping to be successful.

c. From PC-A, telnet to the ISP Lo0 interface and display the NAT table. Pro Inside global Inside local Outside local Outside global

Note: The NAT for the ICMP request may have timed out and been removed from the NAT table.

What was the protocol used in this translation? ____________ tcp
What are the port numbers used?
Inside global / local: ________________ 1034, answers will vary.
Outside global / local: ________________ 23

d. Because static NAT was configured for PC-A, verify that pinging from ISP to PC-A at the static NAT public address (209.165.200.225) is successful.

e. On the Gateway router, display the NAT table to verify the translation Gateway# show ip nat translations Pro Inside global Inside local Outside local Outside global

Notice that the Outside local and Outside global addresses are the same. This address is the ISP remote network source address. For the ping from the ISP to succeed, the Inside global static NAT address 209.165.200.225 was translated to the Inside local address of PC-A (192.168.1.20).

f. Verify NAT statistics by using the show ip nat statistics command on the Gateway router.

Note: This is only a sample output. Your output may not match exactly.

Part 3: Configure and Verify Dynamic NAT

Dynamic NAT uses a pool of public addresses and assigns them on a first-come, first-served basis. When an inside device requests access to an outside network, dynamic NAT assigns an available public IPv4 address from the pool. Dynamic NAT results in a many-to-many address mapping between local and global addresses.

Step 1: Clear NATs.
Before proceeding to add dynamic NATs, clear the NATs and statistics from Part 2.

Step 2: Define an access control list (ACL) that matches the LAN private IP address range.
ACL 1 is used to allow 192.168.1.0/24 network to be translated.

Step 3: Verify that the NAT interface configurations are still valid.
Issue the show ip nat statistics command on the Gateway router to verify the NAT configurations. Gateway# show ip nat statistics

Step 4: Define the pool of usable public IP addresses.
Gateway(config)# ip nat pool public_access 209.165.200.242 209.165.200.254 netmask 255.255.255.224

Step 5: Define the NAT from the inside source list to the outside pool.

Note: Remember that NAT pool names are case-sensitive and the pool name entered here must match that used in the previous step.

Step 6: Test the configuration.
a. From PC-B, ping the Lo0 interface (192.31.7.1 ) on ISP. If the ping was unsuccessful, troubleshoot and correct the issues. On the Gateway router, display the NAT table.

What is the translation of the Inside local host address for PC-B?
192.168.1.21 = _________________________________________________________ 209.165.200.242
A dynamic NAT entry was added to the table with ICMP as the protocol when PC-B sent an ICMP message to 192.31.7.1 on ISP.

What port number was used in this ICMP exchange? ______________ 1 , answers will vary.

b. From PC-B, open a browser and enter the IP address of the ISP-simulated web server (Lo0 interface).
When prompted, log in as webuser with a password of webpass.

c. Display the NAT table.
Pro Inside global Inside local Outside local Outside global

What protocol was used in this translation? ____________ tcp
What port numbers were used?
Inside: ________________ 1038 to 1052. Answers will vary.
Outside: ________________ 80
What well-known port number and service was used? ________________ port 80, www or http

d. Verify NAT statistics by using the show ip nat statistics command on the Gateway router.

Note: This is only a sample output. Your output may not match exactly.

Step 7: Remove the static NAT entry.
In Step 7, the static NAT entry is removed and you can observe the NAT entry.

a. Remove the static NAT from Part 2. Enter yes when prompted to delete child entries.

b. Clear the NATs and statistics.
c. Ping the ISP (192.31.7.1 ) from both hosts.
d. Display the NAT table and statistics.

Note: This is only a sample output. Your output may not match exactly.

Reflection
1. Why would NAT be used in a network?
Answers will vary, but should include: whenever there are not enough public IP addresses and to avoid the cost of purchasing public addresses from an ISP. NAT can also provide a measure of security by hiding internal addresses from outside networks.

2. What are the limitations of NAT?
NAT needs IP information or port number information in the IP header and TCP header of packets for translation. Here is a partial list of protocols that cannot be used with NAT: SNMP, LDAP, Kerberos version 5.

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
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
1900 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)
2801 Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/1 /0 (S0/1 /0) Serial 0/1 /1 (S0/1 /1)
2811 Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
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

Gateway (After Part 2)

Gateway (Final)

ISP (Final)

More Resources

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