Config Lab: Static NAT 1

Wendell Odom
By Wendell Odom October 15, 2021 09:05

Static NAT matches a single inside local address with a single inside global address. It does not conserve addresses, but it does let you make a server reachable to external devices with a permanent address to use with the static NAT entry. This lab asks you to configure static NAT for a small set of servers that need to be made available to users in the Internet.

All about Config Labs

The blog has a series of lab exercises called “Config Labs.” Each lab presents a topology with the relevant initial configuration for each device. The lab also lists new requirements, after which you should create the additional configuration to meet those requirements. You can do the lab on paper, in a text editor, or use software tools like Cisco Packet Tracer or Cisco Modeling Labs.

Once you have created your answer, you can click various tabs at the bottom of this post to see the lab answers, comments about the lab, and other helpful information.

The Lab Exercise

Requirements

Configure static Network Address Translation (NAT) on R1 for devices S1, S2, and S3. The specific rules for this lab are:

  • Configure S1 to use the 172.16.1.10 as an inside global address
  • Configure S2 to use the 172.16.1.20 as an inside global address
  • Configure S3 to use the 172.16.1.30 as an inside global address
  • Use static routes so that R2 can forward packets back to these inside global addresses through a route for subnet 172.16.1.0/24.
  • Assume all router interfaces shown in the lab are up, working, and configured with IP addresses

Figure 1: Static NAT Topology

 

Initial Configuration

Examples 1 and 2 show the beginning configuration state of R1 and R2.

Example 1: R1 Config

 

Example 2: R2 Config

Answer Options - Click Tabs to Reveal

You can learn a lot and strengthen real learning of the topics by creating the configuration – even without a router or switch CLI. In fact, these labs were originally built to be used solely as a paper exercise!

To answer, just think about the lab. Refer to your primary learning material for CCNA, your notes, and create the configuration on paper or in a text editor. Then check your answer versus the answer post, which is linked at the bottom of the lab, just above the comments section.

You can also implement the lab using the Cisco Packet Tracer network simulator. With this option, you use Cisco’s free Packet Tracer simulator. You open a file that begins with the initial configuration already loaded. Then you implement your configuration and test to determine if it met the requirements of the lab.

(Use this link for more information about Cisco Packet Tracer.)

Use this workflow to do the labs in Cisco Packet Tracer:

  1. Download the .pkt file linked below.
  2. Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
  3. Add your planned configuration to the lab.
  4. Test the configuration using some of the suggestions below.

Download this lab’s Packet Tracer File

You can also implement the lab using Cisco Modeling Labs – Personal (CML-P). CML-P (or simply CML) replaced Cisco Virtual Internet Routing Lab (VIRL) software in 2020, in effect serving as VIRL Version 2.

If you prefer to use CML, use a similar workflow as you would use if using Cisco Packet Tracer, as follows:

  1. Download the CML file (filetype .yaml) linked below.
  2. Import the lab’s CML file into CML and then start the lab.
  3. Compare the lab topology and interface IDs to this lab, as they may differ (more detail below).
  4. Add your planned configuration to the lab.
  5. Test the configuration using some of the suggestions below.

Download this lab’s CML file!

Network Device Info:

The CML topology matches the lab topology.

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers

Figure 1: Static NAT Topology

 

Example 3: R1 Config

 

Example 4: R2 Config

Commentary, Issues, and Verification Tips (Click Tabs to Reveal)

Commentary

There are several ways to configure NAT, including static NAT, dynamic NAT, and Port Address Translation (PAT). Static NAT is typically used for one-to-one translations from a specific inside address (called an inside local address) to a specific outside address (called an inside global address). Dynamic NAT differs slightly because it allocates the outside address from a configured pool, with the router choosing the specific IP address to use in each case. The third major type of NAT, PAT (or NAT overload), supports using either a specific outside address or a configured pool.

The difference between PAT and the other types is that the mapping is not one-to-one from an inside address to an outside address. With PAT, there is a many-to-one mapping between the inside local address and the inside global address using unique TCP and UDP port numbers to decide where and how to translate the packets.

For this lab, you were tasked with configuring static NAT, specifically entries to translate the addresses of servers S1, S2, and S3. The requirements also specified which specific outside addresses to use: 172.16.1.10 (S1), 172.16.1.20 (S2), and 172.16.1.30 (S3).

First, you have to determine which interface(s) connect to hosts inside the network and which hosts connect to the outside network. For this lab, R1’s G0/1 interface is connected to S1, S2, and S3 and is considered the inside interface; to configure this, use the ip nat inside command while in interface configuration mode. R1’s G0/2 interface is connected to R2 and is considered the outside interface; to configure this, use the ip nat outside command while in interface configuration mode.

The second task to perform is to configure the static NAT entries. S1, an inside host, uses address 192.168.1.10. R1 then needs an ip nat inside source static 192.168.1.10 172.16.1.10 global command to create the static NAT entry. This causes R1 to monitor packets arriving on the inside interfaces, look for those with a source IP address of 192.168.1.10, and if found, translate that source address to 172.16.1.10 if forwarding that packet out an outside interface. Similarly, the commands ip nat inside source static 192.168.1.20 172.16.1.20 and ip nat inside source static 192.168.1.30 172.16.1.30 create the NAT table entries for the other two servers.

Beyond the NAT configuration, routers in the outside part of the design need to route packets to the inside global addresses. In this lab, the only router in the outside part of the network is R2. The requirements asked that we create a static route for the 172.16.1.0/24 subnet on R2 so that R2 would have a route to send packets back to the inside global addresses. The command to configure R2 would be ip route 172.16.1.0 255.255.255.0 10.10.10.1, with 10.10.10.1 being R1’s G0/2 IP address, as found in the initial configuration R1.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

 

# Summary Detail
1 CPT static NAT show commands differ from real Cisco Packet Tracer (CPT) supports static NAT configuration, and the feature works once configured. However, the show commands display output as if you had configured dynamic NAT or dynamic NAT with PAT, for instance, displaying port numbers in the NAT table entries.
2 Better results in CPT with configure, save, close, open, verify process. CPT may show incorrect NAT behavior if you configure NAT and then move directly to testing. You might want to configure, then save and re-open the .pkt file before testing.

 

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

  1. Verify the NAT configuration by checking the reachability of S1, S2, and S3 from R2 or vice versa.
    1. From R2, try the ping 172.16.1.10 command, pinging S1’s inside global address, which tests the static NAT configuration.
    2. From R2, you could connect to a server with SSH using the ssh -l cisco global-inside-address command.
    3. Alternately, from the command prompt of the servers, try to ping R2 or Telnet/SSH to R2.

More Labs with Related Content!

Config Lab: Syslog 3
Config Lab: Dynamic NAT 1
Wendell Odom
By Wendell Odom October 15, 2021 09:05
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1 Comment

  1. almeidajoaodealmeida November 24, 05:49

    Hi Mr Odom,
    Was R2 included in this lab just for testing purposes, or it has an essential role ?
    Thanks in advance

    Reply to this comment
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