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ISP can tell what my internal network ip's are?

  1. Jul 2, 2009 #1
    ISP can tell what my internal network ip's are???

    To make a long story short, I use a router, with two computers behind it, I called my ISP for technical help, and the technician there told me I use a router (which is a reason for him to not help me..). He said he sees more then one IP address!
    how can it be possible?!

    I suspect he pulled a nasty trick on me - when i use a single computer and connect directly through the modem, (via DHCP, i have an "always on" connection) i can resolve the domain names just fine.
    but through the router i can surf only to the sites which are still in the DNS cache...

    I switched my router to use the OpenDNS servers, and now the problem is over.
    but i got the feeling something fishy took place...
  2. jcsd
  3. Jul 2, 2009 #2


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    Re: ISP can tell what my internal network ip's are???

    No, no one on the outside can see your internal IP addresses. This is because your connectivity device (in this case a router) performs network address translation (NAT). This modifies the header of the packet and inserts the IP address of the router (the public address). The private address class (example 192.168.x.x) is not routable across internet. All outsiders will see is the public address assigned to you by your ISP. They can't know the internal setup unless you volunteer this info.
  4. Jul 2, 2009 #3
    Re: ISP can tell what my internal network ip's are???

    Well, there are ways to guess that you are running behind a router. For example, nmap -O will try to guess the operating system, which will often say something like "Linksys embedded" for a router (assuming your router is visible to the internet, as it would be if you enabled remote administration). This page goes into detail about exactly how nmap makes that guess.
  5. Jul 3, 2009 #4
    Re: ISP can tell what my internal network ip's are???

    what about this site?

    (under the map it tells you your internal IP)

    it uses a javascript silently, which doesn't only display your internal IP address to you, it also registers it in a list on that site.

    is it possible my ISP's DNS redirects me first to a site like this one, runs the script, and then sends me off to wherever i wanted to surf?

    maybe i'll do a traceroute just out of curiosity...
  6. Jul 21, 2009 #5
    Re: ISP can tell what my internal network ip's are???

    No, your router uses NAT (Network Address Translation) to convert the internal to external IPs

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