IP addressing is much more than subnet masks and binary math. In this video, you’ll learn about supernetting, public vs. private IP addressing, network address translation, and automatic IP addressing technologies.
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Hi,
Thanks for the free video series, it’s really very helpful with my studies. I have a question about NAT that I haven’t seen answered in any of the Network+ resources I’ve reviewed, and I’m hopeful that you can answer it for me!
Consider a setup where my computer is on a LAN and has a private ip address of 192.168.1.10 assigned to it. This LAN is connected to a router with the public IP address of 100.0.0.1, to give the network access to the Internet.
Through my computer, I request a resource from the public Internet, for example a web page from http://www.microsoft.com. The router on the network will change the source ip address of the packets that contain the request from my private ip address of 192.168.1.10 to the public ip address assigned to that router (100.0.0.1), and then send the request to http://www.microsoft.com.
When the webserver receives this request, it will send the response back to the ip address 100.0.0.1, and since this is a public routable ip address, it will arrive back at the router for the LAN.
However, how does the router then know which computer on the private LAN to send that response back to? The destination ip address that the webserver will have provided is just the router’s own public ip address of 100.0.0.1. How does NAT keep track of the true source ip address of 192.168.1.10 so that the router can send the response back to the correct host?
Thanks for your help. 🙂
Your router has a NAT table that keeps track of all of the translations that it performs for both the IP addresses and the port numbers. When your router receives the traffic back from the Microsoft web server, it references the table to determine if the traffic is in response to an existing traffic flow. If the traffic is legit, the router NATs the traffic to your internal address.
Thanks for the quick reply. 🙂
when the local pc (in the LAN)sends the data to the router to send it out to the webserver, it goes with a ephemaral port number attached to the IP address of the local pc that remains as an entry in the routing table. When the webserver’s data arrives at the router, this ephemeral port number is compared in the routing table and data is sent to the local IP address owning that ephemeral port number. Hope this helps..and BTW…Prof. messer- YOU ROCK!! THANKS A TON. GOD BLESS.