6.2.7 Packet Tracer - Investigate NAT Operations (Instructor Version)
Instructor Note: Red font color or gray highlights indicate text that appears in the instructor copy only.
6.2.7 Packet Tracer - Investigate NAT Operations
Addressing Table
The following table provides addressing for networking device interfaces only.
| Device | Interface | IP Address and Prefix |
|---|---|---|
| R2 | G0/0 | 10.255.255.245/30 |
| G0/1 | 10.255.255.249/30 | |
| G0/2 | 10.10.10.1/24 | |
| S0/0/0 | 64.100.100.2/27 | |
| S0/0/1.1 | 64.100.200.2/30 | |
| R4 | G0/0 | 172.16.0.1/24 |
| S0/0/0 | 64.100.150.1/30 | |
| S0/0/1.1 | 64.100.200.1/30 | |
| WRS | LAN | 192.168.0.1/24 |
| Internet | 64.104.223.2/30 |
Objectives
- Part 1: Investigate NAT Operation Across the Intranet
- Part 2: Investigate NAT Operation Across the Internet
- Part 3: Conduct Further Investigations
Scenario
As a frame travels across a network, the MAC addresses may change. IP addresses can also change when a packet is forwarded by a device configured with NAT. In this activity, we will investigate what happens to IP addresses during the NAT process.
Instructions
Part 1: Investigate NAT Operation Across the Intranet
Step 1: Wait for the network to converge.
It might take a few minutes for everything in the network to converge. You can speed the process up by clicking Fast Forward Time.
Step 2: Generate an HTTP request from any PC in the Central domain.
a. Switch to Simulation mode and edit the filters to show only HTTP requests.
b. Open the Web Browser of any PC in the Central domain and type the URL http://branchserver.pka and click Go. Minimize the browser window.
c. Click Capture / Forward until the PDU is over D1 or D2. Click on the most recent PDU in the Event List. Record the source and destination IP addresses.
To what devices do those addresses belong?
10.X.X.X and 64.100.200.1 The PC and R4.
d. Click Capture / Forward until the PDU is over R2. Record the source and destination IP addresses in the outbound packet.
To what devices do those addresses belong?
64.100.100.X and 64.100.200.1 The first address is not assigned to an interface. R4 is the second address.
e. Login to R2 from the CLI using the password class to enter privileged EXEC and issue the following command:
R2# show run | include pool ip nat pool R2Pool 64.100.100.3 64.100.100.31 netmask 255.255.255.224 ip nat inside source list 1 pool R2Pool
The address came from the NAT pool R2Pool.
f. Click Capture / Forward until the PDU is over R4. Record the source and destination IP addresses in the outbound packet.
To what devices do those addresses belong?
64.100.100.X and 172.16.0.3. The first address is from R2Pool on R2. Branchserver.pka is the second address.
g. Click Capture / Forward until the PDU is over Branchserver.pka. Record the source and destination TCP port addresses in the outbound segment.
source 80, destination 102x
h. On both R2 and R4, run the following command and match the IP addresses and ports recorded above to the correct line of output:
R2# show ip nat translations R4# show ip nat translations
What do the inside local IP addresses have in common?
They are reserved for private use.
Did any private addresses cross the intranet?
No.
i. Click the Reset Simulation button and remain in Simulation Model.
Part 2: Investigate NAT Operation Across the Internet
Step 1: Generate an HTTP request from any computer in the home office.
a. Open the Web Browser of any PC in the Home Office domain and type the URL http://centralserver.pka and click Go.
b. Click Capture / Forward until the PDU is over WRS. Record the inbound source and destination IP addresses and the outbound source and destination addresses.
To what devices do those addresses belong?
Inbound: 192.168.0.X and 64.100.100.2. Outbound: 64.104.223.2 and 64.100.100.2. The computer and R2; WRS and R2.
c. Click Capture / Forward until the PDU is over R2. Record the source and destination IP addresses in the outbound packet.
To what devices do those addresses belong?
64.104.223.2 and 10.10.10.2, which is WRS and centralserver.pka.
d. On R2, run the following command and match the IP addresses and ports recorded above to the correct line of output:
R2# show ip nat translations
e. Return to Realtime mode.
Did all of the web pages appear in the browsers?
Yes.
Part 3: Conduct Further Investigations
Experiment with more packets, both HTTP and HTTPS and answer the following questions.
Do the NAT translation tables grow?
Yes. There are additional entries as new conversations are started.
Does WRS have a NAT pool of addresses?
No, it uses the same IP address for all devices.
Is this how the computers in the classroom connect to the internet?
It depends on the campus infrastructure. An easy way to check is using something like https://www.whatsmyip.org to determine if all machines in the classroom are using the same address.
Why does NAT use four columns of addresses and ports?
The columns list the inside global, inside local, outside local, and outside global addresses.
Where are the networks are inside global and inside local?
The inside local addresses are on the LANs within each domain. The outside global addresses are from the WAN links to the internet and intranet.
On which devices are NAT services operating? What do they have in common?
WRS, R2, and R4. They all connect internal LANs to outside networks that require routable IP addresses.
Quick Reference Summary
This is a pure exploration lab – NAT is already configured on WRS, R2, and R4 before the activity starts. Use this table to jump straight to the answers without re-reading every step.
| Where | What to check | Answer |
|---|---|---|
| PDU over D1/D2 (Central PC → BranchServer) | Source / Destination IP | 10.X.X.X and 64.100.200.1 – the PC and R4 |
| PDU over R2 (outbound) | Source / Destination IP | 64.100.100.X and 64.100.200.1 – the first address isn't assigned to any interface (a NAT pool address); the second is still R4 |
| R2 | show run | include pool |
ip nat pool R2Pool 64.100.100.3 64.100.100.31 netmask 255.255.255.224 ip nat inside source list 1 pool R2Pool |
| PDU over R4 (outbound) | Source / Destination IP | 64.100.100.X and 172.16.0.3 – the first is from R2Pool on R2; the second is BranchServer.pka |
| PDU over BranchServer.pka | Source / Destination TCP port | source 80, destination in the 102xx range |
| R2 and R4 | show ip nat translations |
Match the recorded IPs/ports above to the correct row |
| Reflection (Part 1) | What do inside local addresses have in common? / Did any private addresses cross the intranet? | They are reserved for private use. / No. |
| PDU over WRS (Home Office PC → CentralServer) | Inbound / Outbound Source & Destination IP | Inbound: 192.168.0.X and 64.100.100.2 (the PC and R2). Outbound: 64.104.223.2 and 64.100.100.2 (WRS and R2) |
| PDU over R2 (Home Office traffic) | Source / Destination IP | 64.104.223.2 and 10.10.10.2 – WRS and centralserver.pka |
| Reflection | Did all web pages appear in the browsers? | Yes. |
| Part 3 | Do NAT translation tables grow? | Yes, additional entries appear as new conversations start. |
| Part 3 | Does WRS have a NAT pool of addresses? | No, it uses the same IP address for all devices (PAT/NAT overload). |
| Part 3 | Is this how classroom computers connect to the internet? | Depends on the campus infrastructure – check with a site like whatsmyip.org. |
| Part 3 | Why does NAT use four columns of addresses and ports? | They list the inside global, inside local, outside local, and outside global addresses. |
| Part 3 | Where are the inside global and inside local networks? | Inside local addresses are on the LANs within each domain. Outside global addresses are from the WAN links to the internet and intranet. |
| Part 3 | Which devices run NAT, and what do they have in common? | WRS, R2, and R4 – they all connect internal LANs to outside networks that require routable IP addresses. |
