Regular Visitor
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11 Messages
bridge mode gets a private IP instead of public and no IPV6 - what's going on!
I've had a comcast modem in bridge mode for 20 years. I know how it works well and I am an IT professional that does networking for a living. My bridge mode stopped getting a public ip/ipv6 address and is instead giving me a private (10.0.0.x) address which is unusable. The folks on the phone are useless - reboot/reset/replace is their script.
The modem of course works fine in gateway mode but that's not the way my network is set up. I wonder if Comcast's dhcp servers are exhausted in my area, I had been moving the modem between several computers in the days leading up to the failure.
I have done the comcast 3Rs one dozen times, I have swapped the cable, attempted (while the technician was at the house) to direct connect a laptop to the modem, all have failed - still no IP.
Has anyone ever seen anything like this and can suggest a fix?
flatlander3
Problem Solver
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1.5K Messages
2 years ago
I've seen that.
My setup? netgear(bridge mode) <--> dedicated firewall <--> Internal subnets. The netgear ends up with a 192.168.100.1 address in bridge mode. Great, that's fine. The WAN port on the firewall will end up with an external public IP address. Cool too. That's how it's supposed to work.
But, there is a race condition that can happen. Reproduction: Reboot the firewall. Reboot the netgear.
IF the netgear is partially up (LAN active) but it fails to actually connect to Xfinity upstream, or hasn't yet (look at the lights on it), then the netgear will assign 192.168.100.2 to the Firewall WAN port -- so same situation. Will it fix itself? On my stuff, sure, eventually. The firewall is waiting for a DHCP renew on the WAN port at that point -- and then it sorts itself out eventually IF the netgear does actually connect back up to Xfinity. How long would that be? Dunno. Hours. Days maybe. I never bothered to look to see what the Netgear passes out in the event Xfinity wasn't there.
So, possible solutions? Have enough backup power on the firewall (your external WAN device/PC/whatever) to ride out power glitches -- can still fail if lease expires while Xfinity is dead. Power controller sequence a startup, either manually or automatically so you start the modem first, then start the internal after a delay (10-15 minutes). I do both and have a failover connection to get to it, cause you know.......Xfinity......
Not bullet proof, but it's kind of a rare event in the first place. What is your cable modem connection doing right now? If it's there, don't reboot the modem, reboot the external WAN/PC/whatever. See if your public IP comes back on it.
Could also be a general DHCP failure or outage upstream from you going on right now, it which case it's just not going to work right now. Your cable is out. (well....your modem could have died too :) Another thought: can you change MAC address on your internal WAN/PC/whatever? Maybe you found a bug on their gear.
(edited)
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EG
Expert
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110.1K Messages
2 years ago
Sounds like the Comcast gateway device has come out of bridge mode for some reason. Maybe because of a recent firmware update. Try confirming which mode it is actually in.
Not sure that this is still valid: https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode
Just a comment. Not sure why you are paying a rental fee if you are using the Comcast gateway device as a straight cable modem only. Why not purchase a compatible cable modem at retail to use with your router. It will pay for itself in fairly short order and perhaps spare you of other heartaches as well. Good luck !
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bobby999999
Regular Visitor
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11 Messages
2 years ago
Thanks for your comments. I do own my modem - comcast used my phone calls to them repeatedly to point out that they are willing to do 'more' if I rented. But regarding knowing I'm in bridge vs firewall mode - I've done that as well. In fact my workaround while I fix this is to keep the modem in firewall mode. When I think I have a solution I reset it to bridge mode, power cycle...and wait.
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