external IP address appearing to bounce between two different IP addresses

Zen Dog asked 2 years ago

I am seeing a strange condition with my external IP address appearing to bounce between two different public facing Comcast IP's when checked every few moments.

Here is the background. I am in a corporate apartment that comes supplied with Comcast internet. I am able to access the modem and configure. It appears this is a Comcast ADSL service and has a Netgear DVDN3700v2 modem / router.

I am trying to port-forward a port to a Slingbox connected on the local LAN. Everything works fine on the LAN but noticed my Slingcatcher cannot access the Slingbox from external networks.

So when I check the ports needed from external network port checker tools, they show as not opened. Even though the port forwarding appears to be configured properly on the router.

I am using whatismyip.com to determine my external IP address and noticed that on random clicks, ever so often, the IP changes back and forth between two different IP addresses. The two addresses have been consistent over several days of checking (always the same two) and they do appear to be legitimate Comcast public facing IP addresses.

Problem is neither of them appear to respond to the router settings I am making. Things like the options to "respond to ping" and "port forwarding" appear to set fine on the router but seem to have no effect on these external addresses. Both IP's respond to pings regardless of my modem settings and port forwarding does not work from either address although appears to be set fine in the router. Other parts of the modem / router appear to make a difference, i.e. if I change ADSL parameters, DSN servers etc. these all respond fine so the issue appears to be on the external facing IP addresses being reported publicly.

Almost acting like a double NAT error response kind of thing but only one router in the network? I have stripped the network down to only the modem / router, the Slingbox and the PC used to check things with. Scanned the network for IP's and only those three devices show as present on the LAN.

Is this bounce between the two public IP address something anyone has seen before? I am somewhat perplexed as to what may be going on.

Is whatismyip service reporting the wrong public IP address? I tried this with several other similar services and get the same results. all report no proxy detected and report the same two IP's bouncing back and forth at random intervals.

Any thoughts or insights?

1 Answers
wimiadmin Staff answered 2 years ago

Is the netgear device supplied by comcast in router mode or in bridge mode? In other words, is the netgear device getting the IP address assigned to it or is the computer getting the public IP assigned to it? If you go to Start, Run, CMD then type ipconfig /all the IP on the computer will show either the same as you're seeing on whatismyip.com or something like 192.168.xxx.xxx. If it's 192, then the netgear device is in router mode....which is ideal.

The next thing I thought of is to make sure the computer you're using to connect to the internet with is only connected to the netgear router supplied by comcast. Make sure it's not connecting to other wireless networks in the immediate area. Also make sure the computer is set to obtain its IP information via DHCP and not getting IP information supplied statically.

What happens if you connect a mobile device via wireless and go to our homepage? https://www.whatismyip.com/ Does it show the same IP info as what you're seeing on the computer? What if you hit refresh? Does the IP change as it does on the computer?

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