Without network address translation (NAT) or port address translation (PAT) you probably wouldn’t be able to access the internet from your computer or at least you’ll be the only one in the house having internet access…in this lesson I want to give you an explanation of why and how we use NAT/PAT for Internet access.
Let’s start with a topology:
On the left side we have a computer on our LAN with the IP address 192.168.1.1 connected to a router. From our ISP we got the IP address 220.127.116.11 and there’s a server on the Internet using IP address 18.104.22.168. If our computer send something to the server what would be the source and destination IP address of the IP packet it will send?
The source IP address will be our computer and the destination IP address will be the server as you can see in the IP packet in the picture above.
Once our server responds it will create an IP packet specifying the computer’s IP address as the destination and the source IP address will be its own IP address.
Is there anything wrong with this example? No, it’s perfectly fine except for one detail…the IP address of the computer and the IP address on the router are private IP addresses. Private IP addresses are meant for our LANs and public IP addresses are for the Internet.
This time we are going to configure NAT (Network Address Translation) and see what the difference is…