Config cisco router NAT to access internet

Teaching Guide: How to Configure Cisco Router NAT to Access the Internet

Level: Beginner (Zero IT Knowledge)

What is Cisco Router NAT?

Cisco router NAT (Network Address Translation) is a way to let devices on your private network (like your home or office) connect to the internet using a single public IP address. Think of NAT as a translator—it changes your private IP addresses into a public one that the internet understands. In this guide, you’ll learn how to set up Cisco router NAT step-by-step so your network can access the internet.

See Step-by-Step Configuration for the commands.
Check How to Test It Works to verify internet access.
Try the Practice Task for hands-on learning.

Step 1: Enter Configuration Mode

Start by accessing the router:

enable
configure terminal

What it does: Gives you permission to change settings.

Step 2: Configure Interfaces

Set up the inside interface (connected to your private network):

interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
 no shutdown

Set up the outside interface (connected to the internet):

interface Serial0/0/0
 ip address 203.0.113.1 255.255.255.252
 ip nat outside
 no shutdown

What it does: ip nat inside marks the private side, and ip nat outside marks the internet side.

Step 3: Set Up NAT Overload

Enable NAT to share the public IP:

ip nat inside source list 1 interface Serial0/0/0 overload

Define which IPs can use NAT:

access-list 1 permit 192.168.1.0 0.0.0.255

What it does: overload lets many private IPs share one public IP (203.0.113.1).

Step 4: Add a Default Route

Tell the router where the internet is:

ip route 0.0.0.0 0.0.0.0 203.0.113.2

What it does: Sends all unknown traffic to the next router (e.g., ISP).

Step 5: Save Your Work

exit
end
write memory

Full Config Example

enable
configure terminal
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
 no shutdown
interface Serial0/0/0
 ip address 203.0.113.1 255.255.255.252
 ip nat outside
 no shutdown
ip nat inside source list 1 interface Serial0/0/0 overload
access-list 1 permit 192.168.1.0 0.0.0.255
ip route 0.0.0.0 0.0.0.0 203.0.113.2
exit
end
write memory

Practice Task

Set up a PC with IP 192.168.1.2 (gateway 192.168.1.1).
Connect it to GigabitEthernet0/0 on the router in Packet Tracer.
Simulate the internet with another router (IP 203.0.113.2 on Serial0/0/0).
Ping 203.0.113.2 from the PC to test internet access.

How to Test It Works

From the router:

show ip nat translations

Look for private IPs (e.g., 192.168.1.2) mapped to 203.0.113.1.
From the PC, ping the internet:

ping 203.0.113.2

Success means Cisco router NAT is working!

Why Cisco Router NAT Matters

Saves public IP addresses by sharing one.
Connects private networks to the internet securely.