Dynamic DNS updating IP address that doesn't match router's IP

[polyrob]

This may have been answered elsewhere but I couldn't find a similar question.

I'm not talking about NAT here.  I understand my devices have different IP addresses that are issued out by the router (ie 192.168.1.n) but my confusion isn't there.  The problem seems to be that my "public" IP address doesn't match the routers actual IP.

On the admin page, the advanced tab shows the IP address of my router (100.65.140.24). This seems to be my REAL ip address, as i can connect to my home server via port forwarding outside my home network when using that IP address.   However, if i use services like http://www.whatsmyip.org then it shows my IP address differently (199.87.86.198). WHen i've set up the dynamic DNS with No-Ip, this is the same address it pushes... so it doesn't work.

I don't understand why I seem to have a router IP and an "incorrect" public IP.  What's going on there?  Does it have something to do with my ISP - and if so, how does my router admin screen know the correct address? Could this be related to IPv6 or some other setting on my router?  Is there anything I can do to fix this?

Thank all!

[TheEther · Accepted solution]

Yes, there two layers of NAT, one by your router and one by the ISP.  And, yes, that means that Dynamic DNS won't work.  One way to work around that is to connect your home server to a VPN provider.

You would have to check with your ISP about the nature of your IPv6 address.  If it's fairly stable, then there are Dynamic DNS providers out there that support IPv6.

[TheEther]

You may think you are not talking about NAT, but your situation actually involves NAT.  Your ISP is doing Carrier Grade NAT.  

You may be wondering, "Why is my ISP doing NAT?"  You see, there are actually no more public IPv4 addresses available.  Unfortunately, your ISP doesn't have enough public addresses to give one to every customer, so they have to resort to using NAT.  Your ISP actually posted about this in their blog (link).

IP address 100.65.140.24 belongs to subnet 100.64.0.0/10, which is a private address range set aside for ISPs to perform Carrier Grade NAT (CGN).  See IETF RFC 6598 (link).  Your ISP assigned it to your router but it is not a public IP address.

IP address 199.87.86.198 is a public IP address.  Technically, it doesn't belong to you.  Rather, it's shared by you and everyone else in your ISP's NAT domain.

The one mystery is why you can connect to your home server using 100.65.140.24.  100.65.140.24 is not reachable from the Internet.  One possibility is that you are accessing it from somewhere else inside your ISP's NAT domain, like a neighbor's place who also has the same ISP.

Moving to IPv6 will certainly get you out of the CGN mess.

Hope this helps.

[polyrob]

Hey Ether,

Thanks a bunch for the reply. I sorta get the reason for NAT and I thought I at least understood it at my router level. It sounds sorta like there is just another level at my ISP... or something like that.

I also see what you're saying about it being unreachable.  I think its working as you'd expect. What confused me was that when seeing my private address as the IP target in my noip configuration then my port was reachable - but you're right, it's only on my network.

So this makes more sense now. But does that mean that given my ISPs setup I can't use a dynamic dns service like No-IP?

My IPv6 address isn't static, right? Is there a way to configure a no-ip/like solution with IPv6?  On No-IP i see a AAAA type but it requires be to give both a IPv4 and IPv6 address. I tried it and when the updates happen it still changes the IP target to my public IP which as we discussed can't be routed to my particular network/port.

Any ideas? Long story short, I'm just trying to set up a dynamic dns mapping to my home server. Thanks again!

[TheEther · Accepted solution]

Yes, there two layers of NAT, one by your router and one by the ISP.  And, yes, that means that Dynamic DNS won't work.  One way to work around that is to connect your home server to a VPN provider.

You would have to check with your ISP about the nature of your IPv6 address.  If it's fairly stable, then there are Dynamic DNS providers out there that support IPv6.

[BARNES]

Dang, I learn something new everyday in networking! Number one reason I enjoy networking!

If that was my situation, I would probably just buy a single static ip from my isp... Without checking I'm pretty sure you could setup your no-ip domain names using a static, else remember that ip likes its your phone number or buy a qualifing domain name and point to your home network.