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http://checkip.dyndns.org/ gives me a different number than what I see when I type in ifconfig in Terminal. The site also gives me a different address than what I see when I run ipconfig in the command prompt on my Windows machine.

I checked these addresses on my computers at work.

What does this mean?

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2 Answers 2

up vote 5 down vote accepted

It means that your computer is on a private network which performs NAT between the private network and the Internet.

NAT is a method to deal with the shortage of IP addresses. Your company might have 100 computers, but only 1 IP address on the Internet (a "public IP address" or "external IP address", in more technical jargon). The way this is handled is the private network at your work gives all the computers a private IP address (usually 10.*.*.*, or 192.168.*.*), and then the router that manages the network "translates" these IP address back and forth from the 1 Internet IP address when the work computers try to access the Internet.

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Much better response than mine. –  viking Jun 16 '11 at 21:56
    
@viking You understood the question and knew the answer - the rest is just practice with communication and writing. Keep answering questions and edit old answers to make them more readable and improve their quality as you improve, and you'll quickly become a valued member of the community :) –  Darth Android Jun 16 '11 at 22:03
    
That's why I'm here. –  viking Jun 16 '11 at 22:12

I'm not sure what you mean by "I Checked these addresses on my computers at work", but typically the ip address used interior to your local network will not be the same as what the world sees outside the network. For example, if you are using a router to connect to the internet, the "world" sees you as the IP address of the router, not your local IP address. See Wikipedia for more information on NAT.

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Just for illumination: There may be thousands, millions even, of people whose IP address for their personal computer is 192.168.1.X. This doesn't cause problems because they are private IPs only really used on private networks behind a device that has a public IP for internet access. –  music2myear Jun 16 '11 at 21:52
    
Actually, it causes these problems, problems when one uses tunnelling to other networks, and the problems alluded to in RFC 1918 § 4. –  JdeBP Jun 17 '11 at 15:24

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