Router requesting additional public IP addresses?


  • About a week ago, I noticed something strange with my entire internet connection. Every device using the internet appeared to not have an internet connection even though it was showing that they were connected to my wifi network. So I rebooted the cable modem and the AmpliFi system, just to be sure. When everything was rebooted, all devices had working internet service again. Good. Now the strange part. A couple of devices had the time change on them going back 1 hour. These devices were setup to adjust date and time based on IP location so when I take them on the road, the time auto-adjusts based on where I am. Yes, I know that I can manually set the date/time to a specific time zone, but it is nice to have it auto-adjust.

    So as I was looking into this, I realized that the public IP address that AmpliFi was showing in the app, on the display of the router, and using several websites to show my IP address all matched. It was an IP address that belonged to my ISP, but the geolocation said that I was in a town across the state from where I really was. There is a time zone change in the middle of our state (South Dakota), so this explains why the devices thought I was on the different time zone. I contacted my ISP about it and they say there is nothing wrong with the network as it is one of their IP addresses. I connected a desktop computer directly to the cable modem and checked the IP address again using ipconfig and also several website that show your IP address and these showed a different IP address from what it was showing when connected to the AmpliFi (ipconfig matched the websites when connected directly to the modem). While still talking to my ISP about this, they could see that the modem had two IP addresses assigned to it, but again said their is nothing wrong with their network. They advised me to contact AmpliFi.

    I contacted AmpliFi through the support section in the app and included the log file. After two emails, I was told that the router is working properly and that was the end of the discussion.

    So, long story short, here is my question. Is it normal for a router (any router) to pull a separate public IP address? I am using a "normal" home service cable connection without a static IP assigned to my service. I did not change any settings before rebooting and the most recent firmware was installed 2-3 weeks before this happened. So again, the modem has 1 IP address showing when connected directly to it and when the AmpliFi is connected to the modem, a different public IP address is assigned and used by all devices connected to the AmpliFi.


  • I also had a strange behaviour this morning. All my devices stopped communicating with the internet even though it had an IP. I rebooted and the IP totally changed from something like 151.x.x.x to 94.x.x.x. I should have taken a screenshot and having no access to logs I can’t confirm the change.

    I am using NowTV/Sky broadband in DHCP mode.

    (I am in 3.1.0rc0)


  • @jason-meyer Do you have multiple devices plugged into the modem? It is normal for your public IP address to change, if you wish to have a static IP your ISP may be able to assign you one.


  • @ui-jt I know that my IP address will change since I do not have a static IP assigned to me by my ISP. My connection is coax to the modem, single ethernet cable from the modem to the Amplifi. The problem/question is this. When the AmpliFi is connected, my public IP address is showing 64.x.x.x. Without powering down anything, I take the ethernet cable from my desktop computer out of the AmpliFi and plug that directly into the cable modem (removing the ethernet cable that goes to the AmpliFi WAN port as the modem only has 1 ethernet port on it) and now the IP address on the desktop shows that my public IP address is 24.x.x.x. When I asked my ISP about this, they said that they could see the modem is pulling 2 IP addresses from them. When I put the cables back as they were, still powering nothing down, the AmpliFi again shows my public IP address as the same 64.x.x.x.

    Why would I get a different/second public IP address just by connecting the AmpliFi to the modem? Shouldn't it be using the existing public IP address assigned to the modem from my ISP?


  • @jason-meyer Is this a modem router combo? I think this would be a question to ask your service provider, why are they assigning different public IP address to your modem.


  • @ui-jt It is a stand-alone modem. It is an ARRIS SURFboard SB6141.


  • @jason-meyer Interesting, I would consult the service provider. Ask how they are handing out public IP's.


  • The computer and the router have two distinctively different mac addresses. When you swap the ethernet cable around it is changing to the IP address assigned to that mac address. This is very normal, standard behavior for a modem. To get a more consistent result for testing you are going to need to set the router to clone/spoof your mac address of the computer you are doing this with.


  • @alan-james I can give that a try. But why would it have not changed anything before now? I currently do not have the mac address cloned to anything.


  • @alan-james I cloned the mac address of the desktop computer within the AmpliFi router and it is now pulling a "local" public IP address. Thanks for the suggestion.


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