# [SOLVED] Loss of DHCP from Cable modem



## bobm_10 (Aug 15, 2008)

Hello All,

I am have and intermitant problem with my Comcast Internet connection. I have a router downstream from my cable modem. When the connection drops I do have LAN connectivity. I am able to browse to the my router/default gateway. When I check the status of the WAN link in the router config, the router does not have a public IP address. Releasing and renewing the WAN IP yields the same private IP address. I have had Comcast come out and check the line and swap Modems. I have swapped three differnt routers and all experience the same problem when the link is down, unable to obtain a public IP addess on the WAN side. Power cycling the Cable Modem solves the problem each time. Any ideas on how to trouble shoot. Thanks Bob


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## Jason09 (Jan 3, 2009)

*Re: Loss of DHCP from Cable modem*

That is an interesting problem given that both the modem and router have been swapped out. 
Do you notice the problem after doing any specific Internet activities?


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

Thanks for the reply. The only pattern is the dropped WAN IP when I get to the router. 

Bob


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## Jason09 (Jan 3, 2009)

*Re: Loss of DHCP from Cable modem*

You do have it setup such that the modem is connected to the router's WAN port, correct?


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

>You do have it setup such that the modem is connected to the router's WAN port, correct? <

Yes, and when it is working on can releace and renew the WAN port IP many times without error.


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## Jason09 (Jan 3, 2009)

*Re: Loss of DHCP from Cable modem*

What is the make and model of your current modem? Has Comcast been giving you the same brand and model modem again, or have they given you different ones?


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

It is an Arris WBM760. When they rpleace my first modem which I had for about 5 years, my performace douibled! when it works it works well. I was happy until the problem returned. Thanks Bob


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## Jason09 (Jan 3, 2009)

*Re: Loss of DHCP from Cable modem*

How far away is your router from the modem?


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

I have a 2' CAT 5E connecting the two. I have replaced that cable as well. Thanks Bob


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## Jason09 (Jan 3, 2009)

*Re: Loss of DHCP from Cable modem*

Try moving the modem and router as close to each other as possible.


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## 2xg (Aug 5, 2009)

*Re: Loss of DHCP from Cable modem*

Hello,

What's the Make and Model of your router? Mac cloning is required when connecting a router to a Comcast Modem.


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## Fred Garvin (Sep 4, 2010)

*Re: Loss of DHCP from Cable modem*

Bob, I would connect your computer directly to the modem as a test - bypass the router all together. See if you have any problems. I'm assuming this is a Modem only and does not have a router built in.

Unscrew your coax cable going into the modem and check the end connector to make sure it's firmly on the coax. Is the center conductor straight and extending just a bit beyond the connector? Is this Coax line dedicated to the modem or is it split for TV?

If you have the manual for the modem and can find its IP address, you should be able to see some diagnostic info and error logs. Contact your ISP for the IP addy or have them look into it. If the modem is getting errors, they will be logged and may help find the cause.


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

>What's the Make and Model of your router? Mac cloning is required when connecting a router to a Comcast Modem.<
I have two Netgear and one Linksys I have been swapping. I have not not used Mac cloning in the 6 years that I have had Comcast. I am curious, why do I need to use MAC cloning? thanks Bob


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

Hi Fred:

>Bob, I would connect your computer directly to the modem as a test - bypass the router all together. See if you have any problems.<
I have 6 devices on the local LAN relying on the link so any prolonged experiment is difficult. What I have done is connected a laptop directly to the modem when I am experiencing a problem. Since the modem requires a power cycle to recognize the laptop everything works after the cycle, plug the router back in, power cycle and I am still up. 

> I'm assuming this is a Modem only and does not have a router built in.<
Correct.

>Unscrew your coax cable going into the modem and check the end connector to make sure it's firmly on the coax. Is the center conductor straight and extending just a bit beyond the connector? <
This has all been checked out numerous times. The Comcast tech even swapped the connectors.

>Is this Coax line dedicated to the modem or is it split for TV?>
Spilt but tech confirmed a strong signal, in addition my internal coax is real quiet according to his meter. I am not sure how you can ovoid the split anyway, we need to have cable TV. 

>If you have the manual for the modem and can find its IP address, you should be able to see some diagnostic info and error logs. Contact your ISP for the IP addy or have them look into it. If the modem is getting errors, they will be logged and may help find the cause. <
Yes they see the errors but are not sure what is causing it. I should ask them for access, perhaps I can research some of the errors. they say, dropped connection on the WAN side. 

Thanks for your help,


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## 2xg (Aug 5, 2009)

*Re: Loss of DHCP from Cable modem*

*bobm*....Please have a read here, some ISP's requires Mac cloning, Comcast is one of them. Here's the process.


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## Fred Garvin (Sep 4, 2010)

*Re: Loss of DHCP from Cable modem*



bobm_10 said:


> Yes they see the errors but are not sure what is causing it. I should ask them for access, perhaps I can research some of the errors. they say, dropped connection on the WAN side.


Bob, IMO, everything you've said points to a Comcast network problem. It's more than just reading a signal level with an RF meter. The problem could be anywhere upstream to Comcast from your modem. The tech should have been able to get some info from your modem logs which he can report back to the office.

I read the link 2Xg posted and not really sure I buy the guy's explanation. What the tech probably should have done was called in the serial & MAC address of the new modem to Comcast so it's registered to his account. I could be wrong, but that's how it's done around here.


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## bobm_10 (Aug 15, 2008)

*Re: Loss of DHCP from Cable modem*

Hi Fred: I had another service tech come out. We found a fault connection outside at the demarc. The demarc is under my porch so the first time he came out we did not look. This time we removed the floor boards. It has been about 5 days without a problem, keeping fingers crossed. 

The MAC addess problem does not seem to apply to my situation. The Modem is seeing the MAC address of the router, no need to show it the MAC address of one of my devices on the internal LAN.

thanks

Bob


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## Fred Garvin (Sep 4, 2010)

*Re: Loss of DHCP from Cable modem*

Great to hear Bob! Thank you for the update. I'll marked this as solved for now, but post back if the problem comes back.


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