How can a website know my private IP address?

Question

I recently came across this website called whatsmyrouterip.com. How does this website know both my private and public router IP address?

The method is described by and implemented in the code on the page. — symcbean, Jul 13 '19 at 22:59
yeah but that only showed me how to get my router ip by myself, I am asking how does the website know — , Jul 13 '19 at 23:10

score 6 · Accepted Answer · answered Jul 13 '19 at 23:33

It looks like it tries to fetch the web page at each of a list of common router local IPs with JavaScript, deciding that it's found it when one responds. It uses jQuery's $.get(), but there are lots of ways to fetch one a web page from within another¹. This assumes that the router has a web administration panel on port 80, but they usually do.

The relevant code is on and around line 2495 of the source code for the page (view-source:http://whatsmyrouterip.com/):

2463   var ips = [["10.0.0.10", []], ["10.0.0.100", []], ["10.0.2.1", []], ...

       <snip>

2495   $.get('http://' + currentIp[0], function () {
2496     foundIp();
2497   });

¹Well, getting the content may be tricky because of Same-Origin Policy, but a request that fails because of SOP and one that fails because the site doesn't exist look different so you can still tell if there's something there or not.

How can a website know my private IP address?

1 Answers1