bml's profile

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21 Messages

Fri, Nov 6, 2020 4:00 PM

New router: MAC address and DNS issues

Our old router was becoming unreliable, so I purchased a NetGear R6700 v3 as a replacement.

 

During self-install, I found that the router could not connect to the internet via the cable modem using it's default MAC address, but was able to by using the computer MAC address. That is fine on it's own.

 

The second issue is that automatic DNS via the router does not work for any attached device. Instead, all devices need to set their DNS manually (we use 8.8.8.8 and 8.8.4.4). This is quite a nuisance, especially for the less tech-savvy users.

 

I contacted Comcast support with the MAC address of the new router, which they said would be "entered into the system" but that never resolved anything.

 

I'd like to resolve at least the DNS issue. I do suspect these issues are related (but could be wrong).

 

Can someone offer advice on how to proceed from here?

Responses

EG

Expert

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87K Messages

7 m ago

Did you re-boot the modem after connecting the router to it ?

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EG

Expert

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87K Messages

7 m ago

Do it with the router's default MAC addy.
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bml

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21 Messages

7 m ago

Yes, I did reboot the modem.

 

I'd be happy to do it again, although it's not clear if I should do that with the computer MAC address or the router default MAC address.

bml

Frequent Visitor

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21 Messages

7 m ago

As suggested, I did a modem reboot with the router set to it's default MAC address.

 

This did resolve the first problem - the router is now able to connect to the internet via the modem with the default MAC address. Nice.

 

However, it did not resolve the DNS issue. All attached devices still must manually set their DNS to alternate servers - 8.8.8.8 and 8.8.4.4 seem to work fine.

 

I'll keep poking around, but if anyone has another suggestion, I'd be thrilled to hear it.

EG

Expert

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87K Messages

7 m ago

Good to hear ! Do you have the router and the network clients set to *obtain DNS servers automaticaly* ?
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bml

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21 Messages

7 m ago

Yes, I have just reconfirmed that when the router and/or any attached device is set to use automatic DNS servers, they cannot resolve names into IP addresses and effectively are rendered off-line.

 

Only when an attached device is configured to manual DNS (8.8.8.8, 8.8.4.4) can it resolve names and access the internet.

 

FWIW, per the router diagnostics page the automatic DNS servers appear to be (75.75.75.75 and 75.75.76.76), which I assume are Comcast servers. Those do ping fine.

 

For a cursory Google search, it seems that DNS issues are a common problem with Comcast, but it isn't clear to me if I am experiencing the same problem as most (which seem to be intermittent rather than consistent). Some examples (link, link, link).

 

One suggestion from those discussions is to consider if this a connection issue. I can say that it takes about 15-20 min to re-establish a good modem connection after reboot.

 

Power info for the modem (Netgear CM600):

Downstream power: 12.5 - 13.5 dBmV (Ch 1-16), (-19) - (-19.5) dBmV (Ch 17-24)

SNR for all downstream channels is between 38-40 dB

Upstream power: 24.0 dBmV (Ch 1) <- only connected channel

EG

Expert

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87K Messages

7 m ago

The up and down power levels are out of spec. Perhaps it would be best to have a tech out to investigate / correct them. Good luck !

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bml

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21 Messages

7 m ago

Ah, okay.

 

Out of curiousity, what are the specs?

bml

Frequent Visitor

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21 Messages

7 m ago

Whoops - nevermind.

 

Found them here: link

EG

Expert

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87K Messages

7 m ago

From the "Troubleshooting Tips" sticky topic right at the top of this board;

 

 

Specification      Min             Max

Downstream Power Level  
Cable Modems-10dBmV+10dBmV
Gateways and EMTAs-7dBmV+7dBmV
Downstream Signal to Noise Ratio35dB-
Upstream Power Level+35dBmV+50dBmV
Upstream Signal to Noise Ratio31dB-
Upstream Receive Power-2dBmV+2dBmV

 

I am not a Comcast Employee.
I am a Customer Expert volunteering my time to help other customers here in the Forums.
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