NAT the translator and Port forwarding!!

Today, a friend of mine asked me to tell my IP by issuing the ipconfig (ifconfig in GNU/Linux) command so that he could Telnet into my machine from his home. This question of his ignited the whole action of the evening!! It all began with a small explanation to him about about private & public IP addresses, NAT and the fact that IP address given out by ipconfig was that which was assigned to the NIC of the machine by the user or admin.

Today was his first day at a CCNA class, so he wanted to freak out with Telnet. We took it a bit further by trying to Telnet int each other’s machine through the Internet. We first found out each other’s public IP address from and also enabled Telnet service on our routers. We gave a try. Yes!! We were in!! But only upto each other’s routers 😦

It was almost after 2-3 minutes that the necessity of Port Forwarding striked me 🙂 And soon after we configured port forwarding, we were into each other’s machines!! Haha!! That was fun!!

Warning: Don’t keep Telnet ports open except for lab testing purposes. It uses no encryption and you are at a very high security risk running the Telnet service.

Here is a great ‘simple’ explanation of how NAT works, by Cisco,
Find how to configure your router for port forwarding here

Image courtesy: Cisco Systems, Inc.

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