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is it possible to get my nat ip address across the Internet? I know I can see my ISP address but not my internal address. I remember seeing this done one time but I am thinking it was a java applet and not just a web page. can it be done with a regular web page?


edit for clarification: I want to know if there is a way someone can get my local ip on my internal network from the Internet.

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You want to find the IP address you have on your local network? Or do you want to know whether someone else can find it? –  innaM Oct 30 '09 at 15:15
Thanks for the clarification. –  innaM Oct 30 '09 at 15:33

3 Answers 3

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Hi, is it possible to get my nat ip address across the Internet?

By normal means no. Your internal address will be mapped to your router public address and the information lost to anyone outside. When they connect back to you (being an active connection or a response packet from a connection you established), your router will remap the public address back to your internal IP address (making also any necessary changes to the packets contents) without the outside knowledge.

However, if someone from the outside somehow gains access to your router translation tables (where the mappings and connection states are kept), then indeed they can learn about your internal IP.

They can also learn about your internal IP if you actively advertise it to those you connect to. This can be happen in numerous ways. An application may simply read your internal IP from your network device and forward it along when connecting. And this is how probably you've seen this happen. With Java enabled, for instance, a browser can unwillingly and without your knowledge send your internal IP by running a malicious webpage script.

Note however that there are also legitimate uses for this, like recording internal and external IPs for purposes of abuse control and reporting.

However, the internal IP is of no practical use to anyone outside. They can't use it to access your machine. So this shouldn't worry you in any way.

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The router is the NAT boundary in most home setups. it is transparent to the ISP, so "access to your ISP router translation tables" don't matter, they would need the NAT table in your personal router. If they can get that then they can already get your ip address. –  MDMarra Oct 30 '09 at 15:49
Actually, I wasn't thinking in home or any other network in particular. Wherever the router that does the IP mapping is, it is where the Translation tables are located. But I will edit to clarify. –  A Dwarf Oct 30 '09 at 16:09
Done. It was a simple matter of removing the word "ISP". –  A Dwarf Oct 30 '09 at 16:10

edit for clarification: I want to know if there is a way someone can get my local ip on my internal network from the Internet.

As often the answer is "it depends". Besides the possibilities of Java and other applications mentioned by @A Dwarf, from pure network/TCP perspective you are basically asking if someone can detect that you are using NAT. So the answer is no; if this was possible you would not have to use NAT in the first place :)

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As far as I know, only if they can get into your router setup or if you have NAT / virtual server on the router that tells an external request which internal IP to use on a given port.

Other than that, the router is the only thing that the outside world can see and it has your external IP. I'm sure that someone can hack a router for this info though.

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