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My laptop is getting both a valid IP address and the broadcast address on this network

Posted on 2016-11-25
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Last Modified: 2016-11-29
I've had a long standing issue at a client site where there is a Comcast gateway, a couple of Ubiquiti access points and a security system with DVR. When I connect my laptop to this network, I am unable to access the Comcast gateway until I first disconnect the DVR, then reboot the gateway. At that point, I can access the Comcast gateway web GUI at 10.1.10.1. When I reconnect the DVR, I again lose access to the Comcast gateway. The network works fine otherwise. While I lose access to the gateway on the private side of the device, Comcast is able to access it fine from the WAN. I always assumed it was a problem with the DVR. Recently, both the Comcast gateway was replaced because of a dead port and the DVR was replaced because a power surge took it out. I was called out to put the new DVR IP address in the DMZ of the Comcast gateway, which I did. Even after both devices have been replaced, I still have this issue with accessing the web GUI of the Comcast gateway. This time I ran an IP scanner (Advanced IP Scanner from Famatech) and found that my laptop was getting BOTH a valid IP address on the 10.1.10.0/24 subnet AND ALSO the broadcast address at 10.1.10.255. Both addresses are associated with the MAC address of my laptop. I have looked at a lot of networks over the years, but I have never seen a device get the broadcast address for the network. Is this a clue that may be related to the web GUI access issue? Ideas on how to troubleshoot this? Or is this maybe a fluke of the IP scanner?
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Question by:tcianflone
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by:John Hurst
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my laptop was getting BOTH a valid IP address on the 10.1.10.0/24 subnet AND ALSO the broadcast address at 10.1.10.255.

One (first) is the IP address and the second is the Gateway. That should be fine.

Give you DVR a static IP address on your network.

Make sure your laptop is set for DHCP and do a TCP/IP reset / DNS Flush.

Open cmd.exe with Run as Administrator
Then  netsh int ip reset c:\resetlog.txt
Also, ipconfig /flushdns followed by net stop dnscache followed by net start dnscache

Now shut down both devices, start them back up and test.
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giltjr earned 500 total points
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When you do a ipconfig /all does it show the ".255" address?  Some operating systems and devices will respond  to ping's of the broadcast address.  So just because your device responds to a ping of the ".255" address does not mean that address is assigned to it.
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by:tcianflone
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Touche, giltjr! I just tested this in my office. My mistake was to set the range of the IP scan to include the .255 address and of course the laptop responded. But, indeed, the laptop was only assigned the one valid address, not both. So this has nothing to do with the mystery of accessing the Comcast gateway GUI at this site. Still baffled by that, but I can only assume now that is has something to do with the configuration of the DVR.
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by:giltjr
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Do you have a small switch?  If so connect your Comcast gateway to it, your PC to it and make sure you can access the Comcast gateway.

Then connect the DVR and see what happens.  If you lose access to  the Comcast gateway, disconnect it, wait a minute or two then run the IP scan tool.  See if by chance the DVR is taking over the IP address of the Comcast gateway.

If so, then you need to assign it a static IP address, as John Hurst suggested, and connect the Comcast gateway again.
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by:tcianflone
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Yes, there is a switch there and yes, connecting the DVR is definitely what kills access to the Comcast gateway web GUI. I've tested that several times. I agree the DVR should have a static address and that is how I would have set it up, but I have to get the company that manages it to agree with me. Thanks for the follow up. I will attempt to work with the security company to resolve this.
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by:giltjr
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I would disconnect the Comcast gateway, wait a minute or two and then ping 10.1.10.1.  If it responds then my guess is that the DVR is configured to use the address 10.1.10.1.
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by:tcianflone
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Yeah, that's always been my guess, but the person that manages it swears there is no way that's happening. And with DHCP, it shouldn't. But in this case I will have to confirm everything myself. Thanks again for the follow up.
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by:giltjr
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Although I would not expect it to happen "today", but years ago we had a network attached printer that would hit a bug in it's firmware, re-boot itself, and take over the IP address of our network's default router.  So you never know.
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