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My Router's WAN IP is a public IP - 172.100.x.x And my public IP is - 182.x.x.x

Why do I have 2 public IP's? Does this probably mean that my router is behind another router? But then what is the use of configuring 2 public IP's?

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    In order to answer this we'd need to know more about your setup. Where did you find this public IP? Are you certain that you where connected to your network when you found it or that you aren't using a VPN. 172.100.x.x is a Time Warner Address, 182... is not.
    – Preston
    Jul 19 '15 at 20:14
  • In router status, the IP address is - 172.101.6.207. When I type "Whats my IP" in google it is - 183.87.43.186. Jul 20 '15 at 20:48
  • Is this about a home network? Home networking is off-topic here. You can ask over on Super User Aug 10 '15 at 14:39
  • Are you using usb dongle attached to the router. I have the same issue when i conèct to net usin huawei usb dongle attached to asus rt ac68u router usb.
    – user25500
    May 19 '16 at 3:11
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I can see two possible explanations:

  1. Your WAN address is your public address, but you are yourself using something that makes it look like you are accessing the internet somewhere else. One example is a proxy server which uses the 182-address to access the internet. A website telling you what your public IP is would see the 182 address. Often these sites will also tell you there is a proxy in play if the proxy sets the HTTP X-Forwarded-For header. Another possibility is a (VPN-)tunnel.

  2. Your router is indeed behind another router performing address translation. Chances are your internet provider is doing carrier-grade nat. As to 'why?', ISPs do this to overcome an IPv4 address shortage. This way, they can provide internet access to many customers using just one public IP.

Assuming you are not using a proxy, something's off... It is indeed strange that a 172.100 address is being assigned to customers, which are then hidden behind some 182 network. 172.100 was assigned by ARIN, 182 by APNIC...

Assigning RFC1918 private addresses to customers when using carrier-grade NAT can cause problems when the same network is in use inside the customer's network. The right thing to do (according to RFC6598) would be using addresses from 100.64.0.0/10. Using (parts of) 172.100 avoids possible conflicts with private addresses, but could make (parts of) the hosts rightfully using 172.100 addresses unreachable for you. Also, using 172.100 creates a risk of these routes being leaked into the global BGP table.

This is speculation, but since your WAN IP starts with 172, someone could be assuming the whole 172.0.0.0/8 range is private (but actually only 172.16.0.0/12 is private). Otherwise, someone is knowingly squatting IP space.

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    It is far too common for people to assume what's "private". I've seen MANY places using non-1918 172.x space. (my current employer being one of them)
    – Ricky
    Jul 20 '15 at 5:45
  • @Gerben Thanks for the answer. But I have 3 questions - 1. NAT is used to map Private IP's to Public IP's only right? Because if we are mapping Public IP's to Public IP's, it serves no purpose. 2. In Double NAT, the router's LAN ports have private IP's. But do the WAN ports also have private IP's? 3. Similarly can we have triple, quadruple NAT's? And can multiple NAT solve IP address exhaustion theoretically? Jul 20 '15 at 20:56
  • @sahil_time, if you have follow up questions, you may want to consider asking them as new questions. For #1, no NAT is used to map IP to IP. It can be private to public, but it could be private to private, public to private, or public to public. It could be that your ISP is "re-using" their PI IP addresses internally behind different CGN gateways. #2, see #1. #3, the more NAT that takes place the more likely problems will result and the more performance is often degraded.
    – YLearn
    Aug 12 '15 at 2:28
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There is another reason why this can occur and that is simply if you are using a 4G or 5G router (rather than a standard modem/router that connects an RJ45 ethernet cable to the incoming broadband connection at the wall or in the US this is sometimes a coaxial connection PLUS a wall plug to power it - a 4G/5G modem/router will only have the wall plug for power).

If this is the case then your 4G or 5G modem/router is connecting over the air to your ISP's physical servers first - the WAN address reported by the router - and then that server connects to the internet where a public WAN address is assigned to the imternet requests at the ISP's server.

Technically a 4G/5G router has a script sent over the 4G/5G cellular network called the APN - access point name - with your cellular networks handshake and connection settings to the ISP's physically located server that then connects to the internet. The first connection to the ISP's server reveals the IP address of that server to your modem/router as the WAN address that it has connected to.

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