What is this private LAN address accessible through my ISP?

Recently some websites I regularly visit (most recently docker.com, atlasobscura.com; both of these sites return an issue with a fortinet-ca2 certificate) have started showing certificate issues on my computer. I thought this is some mix-up so I decided to ignore the certificate warning issued by my browser.

When I did that, a different address showed up in the address bar of my browser, also showing the certificate error (also fortinet-ca2 issue), with the IP 10.0.200.5 in it. I haven't tried ignoring this certificate issue past this point. My home LAN addresses are 192.168.0.x, so this was odd so I tried pinging the IP address. The ping was successful. Traceroute returns the following:

Tracing route to 10.0.200.5 over a maximum of 30 hops

  1     2 ms     7 ms     8 ms  192.168.0.1
  2    11 ms    18 ms    11 ms  [address and IP on my ISP]
  3    12 ms     8 ms    12 ms  10.0.200.5

Trace complete.

Any idea why I can access some private LAN address through my ISP? Also, what's with the redirection when I choose to ignore certificate issues?

Solution 1:

This sounds like a firewall problem. Your firewall (or your ISP) is performing SSL content inspection on your internet traffic and is presenting the firewall's default SSL certificate to your browser. You would typically install this certificate on your end user computers so that they trust the certificate, therefore eliminating the warning. If you're not using a Fortinet firewall then I'd suggest reaching out to your ISP to find out if they're performing SSL content inspection on your internet traffic.

https://docs.fortinet.com/document/fortigate/6.0.0/cookbook/565000/preventing-certificate-warnings-default-certificate