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I've been having some issues lately with an online service, and the vendor suggests it's because I'm behind a dual-NAT. I spoke with my ISP and they say that I am not behind a dual-NAT. Additionally, I have a static IP.

However, my understanding was that if any of the hops at the start of a tracert are private IPs (on the WAN side of my router) then this would indicate dual-NAT.

Here's a tracert I did to Google DNS:

PS C:\Windows\system32> tracert 8.8.8.8

Tracing route to google-public-dns-a.google.com [8.8.8.8]
over a maximum of 30 hops:

  1    <1 ms    <1 ms    <1 ms  DSL-AC68U-B498 [192.168.1.1]
  2    13 ms    11 ms    16 ms  10.0.254.108
  3    15 ms    15 ms    11 ms  10.0.248.41
  4    21 ms    18 ms    20 ms  10.0.248.173
  5    19 ms    19 ms    18 ms  xe-0-7-0-2-2.r00.londen10.uk.bb.gin.ntt.net [83.231.146.153]
  6    18 ms    20 ms    19 ms  ix-ae-24-0.tcore1.ldn-london.as6453.net [80.231.60.122]
  7    21 ms    21 ms    20 ms  72.14.217.89
  8    22 ms    22 ms    47 ms  108.170.246.161
  9    19 ms    19 ms    19 ms  108.170.230.161
 10    18 ms    18 ms    18 ms  google-public-dns-a.google.com [8.8.8.8]

Trace complete.

So the 3 hops outside of my router are 10.0.x.x addresses. Am I dual-NATed ?

Thanks in advance for any advice.

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  • Seems so. You seem to have two routers that are connected one behind the other, each doing its own NAT. See this answer for more info.
    – harrymc
    Aug 31, 2018 at 11:58
  • 1
    Nothing would stop an ISP from routing internet IPs over internal IPs. That being said, please look at your router’s WAN address. There is only one way to retrieve this address: Your router’s web interface. Is this address a public IP address?
    – Daniel B
    Aug 31, 2018 at 12:42

1 Answer 1

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No, private IPs at the beginning of a traceroute are not a sufficient condition for double NAT. They may as well be internal routers without NAT in your ISP's network.

But if you are behind multiple home routers which each do NAT, then you'll of course also see the IP of the home routers doing the NAT, and that usually is a private IP.

In particular, if DSL-AC68U-B498 is your home router, if this router is directly connected to your DSL/fibre/whatever, and if you have a static IP given by your ISP that is reachable from the outside, then your ISP won't do NAT behind your router. Because if there was additional NAT involved, your router wouldn't be reachable, at all.

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