I have a standard network setup where all my network devices, both WIFI and ethernet, are connected to the same router. Lately, however, I've been experiencing some very strange behavior.

It started as a simple connecting error, when I tried to reach an FTP server using the external IP. Of course I went right into one of those CheckMyIP sites, to double check the IP and it turned out to be correct.

Then I went into my router setup, which is through a tool called aiport-tool, because I have an Apple Aiport Extreme router. Turns out the router displays a different external IP, and for some reason that external IP works when I try to access the FTP server.

Can anyone explain what is going on? Why are the devices connected to the router displaying an incorrect external IP?

BTW i have no VPN/proxy setups on any of my devices.

  • Your external IP address might have been changed by your ISP and you might be loading a cached version of an IP website. The fact that the IP reported from your Airport loads without issue leads me to believe that is now your external address. – Trey Nov 4 '13 at 17:47
  • If you look up the two addresses in the database of such sites as whatismyipaddress.com, do both of them belong to your ISP? If so, the most likely explanation is that the ISP changed your address without informing you (should he have? Do you have a contract for a static public IP? If you are not paying extra, then your ISP is changing your address to prevent giving you what you should be paying for). – MariusMatutiae Nov 4 '13 at 17:51
  • That sounds reasonable. I don't pay for a static IP, and i i'm pretty sure that my ISP changed my IP without any notice. Thanks! – Martin Nov 4 '13 at 18:36
  • In the question it is not clear what role does a particular external IP address play in accessing the FTP server. Do you have to configure an access list on the FTP server? – pabouk Nov 4 '13 at 19:29

if your internet provider uses CGNAT http://en.wikipedia.org/wiki/Carrier-grade_NAT you will get this problem.

Check with your ISP if they are using CGNAT.

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