Answers

Figure 1: Dynamic NAT Topology

interface GigabitEthernet0/1
 ip nat inside
!
interface GigabitEthernet0/2
 ip nat outside
!
ip nat pool nat_pool 172.16.10.10 172.16.10.20 netmask 255.255.255.0
ip nat inside source list 1 pool nat_pool
!
access-list 1 permit 10.1.1.0 0.0.0.255

 Example 3: R1 Config

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 rather than pre-determining the specific address to use in each case. The third major type of NAT, PAT (or NAT overload), uses 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.

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.

Next, you were tasked with configuring a NAT pool called nat_pool and including addresses 172.16.10.10 through 172.16.10.20. The command ip nat pool nat_pool 172.16.10.10 172.16.10.20 netmask 255.255.255.0 global command does just that. The two addresses define the beginning and ending range of addresses in the pool.

Note that the netmask value (255.255.255.0 in this example) acts as a math check. If the two addresses are in the same subnet, then your answer is correct if you use the listed subnet mask. It does not have to match the actual subnet mask. In this case, masks 255.255.255.224, 255.255.255.192, and 255.255.255.128 would also work.

Take a quick look at the ip nat inside global command; note that it refers to an ACL (ACL 1 in the sample). The ACL should match the inside local addresses that should drive the NAT function. In this case, packets coming from hosts in subnet 10.1.1.0/24 should cause dynamic NAT to occur, so the simple ACL 1 matches packets whose source IP addresses are in that subnet with the access-list 1 permit 10.1.1.0 0.0.0.255 command.

Taking a closer look at the ip nat inside command, the fourth and final task requires this command to link the ACL logic of matching packets coming in a source interface (source list 1) to the pool to dynamically allocate an inside global address (pool nat_pool). The full command strings all the ideas together: ip nat inside source list 1 pool nat_pool.

Finally, note that the requirements tell you to configure static routes on R2 as needed for the inside global addresses. In this case, the configuration uses a range of addresses in the subnet between R1 and R2, so R2 already has a connected route that includes the addresses used by NAT. So there is no need for any additional static routes.

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:

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 dynamic NAT configuration by checking the reachability from S1, S2, and S3 to R2 using ping.
   1. At router R1, using the show ip nat translations command to verify that the NAT table begins with no entries.
   2. From S1/S2/S3, try the ping 172.16.10.2 command, pinging R2’s global address, which tests the static NAT configuration.
   3. At router R1, using the show ip nat translations command to verify a new NAT table entry appeared in support of each flow.
   4. Note that the entries timeout quickly with CPT.
2. Use a TCP or UDP app to test from S1/S2/S3 to router R2. For instance, use Telnet.
   1. Because the initial configuration did not prepare router R2 for inbound Telnet, add configuration on R2 as follows:
      • line vty 0 15
      • transport input all
      • login
      • password cisco
   2. From S1/S2/S3, try the telnet 172.16.10.2 command, pinging R2’s global address, which tests the static NAT configuration.
   3. At router R1, using the show ip nat translations command to verify a new NAT table entry appeared in support of each flow.