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

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Discussion Overview

The discussion revolves around whether an Internet Service Provider (ISP) can see the internal IP addresses of a user's network when they are behind a router. The conversation touches on technical aspects of networking, including Network Address Translation (NAT) and potential methods for external entities to infer internal network configurations.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses concern that their ISP can see multiple IP addresses when using a router, questioning how this is possible.
  • Another participant asserts that ISPs cannot see internal IP addresses due to NAT, which modifies packet headers to show only the public IP address.
  • A different participant mentions that while ISPs cannot see internal IPs, tools like nmap can infer that a device is behind a router based on network characteristics.
  • One participant raises a concern about a specific website that claims to display internal IP addresses using JavaScript, questioning if ISPs could redirect users to such a site to gather information.
  • Another participant reiterates that NAT is responsible for the conversion of internal to external IPs, reinforcing the idea that internal addresses remain hidden from the ISP.

Areas of Agreement / Disagreement

Participants generally agree that ISPs cannot directly see internal IP addresses due to NAT. However, there is some debate regarding the implications of external tools and websites that may infer or display internal IP information.

Contextual Notes

The discussion includes assumptions about the visibility of network configurations and the functionality of NAT, but these aspects remain unresolved in terms of specific scenarios or exceptions.

Who May Find This Useful

Individuals interested in networking, privacy concerns related to ISPs, and those curious about how internal and external IP addresses interact may find this discussion relevant.

fargoth
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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...
 
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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.
 


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.
 


what about this site?
http://www.auditmypc.com/anonymous-surfing.asp

(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...
 


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

j
 

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