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Question Modem Needs Restart When Switching PC's

tron9901

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Dec 13, 2009
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My router got fried from a lightning strike and it will be a while before I get a new one. Until then I have to switch my Comcast modem's ethernet cable between my laptop and desktop PC. Connecting to the laptop is seamless, no modem restart required. But every time I connect to the desktop, I have to power cycle the modem in order to get internet connectivity.

The desktop network adapter shows the message "Ethernet doesn't have a valid IP configuration." I've tried all the commonly known commands which are supposed to fix this:
netsh winsock reset
netsh int ip reset
netcfg -d
ipconfig /release
ipconfig /renew
ipconfig /flushdns
ipconfig /registerdns

The above commands never work and I always have to power cycle the modem. Any idea why? And why this only happens on the desktop and not the laptop?

Thanks in advance.
 
This is normal. Your modem will lock onto the mac address of the connected device and won't let go until it's usually restarted.

What you can do in the meantime is get the comcast equipment for a month or two as it has a built in router so you don't have to keep switching cables like this.

If you plan to only use wired, you can probably pick up a cheap router that is gigabit and weak on the wifi side to use for a while.
 

Bassplate

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Apr 4, 2020
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My router got fried from a lightning strike and it will be a while before I get a new one. Until then I have to switch my Comcast modem's ethernet cable between my laptop and desktop PC. Connecting to the laptop is seamless, no modem restart required. But every time I connect to the desktop, I have to power cycle the modem in order to get internet connectivity.

The desktop network adapter shows the message "Ethernet doesn't have a valid IP configuration." I've tried all the commonly known commands which are supposed to fix this:
netsh winsock reset
netsh int ip reset
netcfg -d
ipconfig /release
ipconfig /renew
ipconfig /flushdns
ipconfig /registerdns

The above commands never work and I always have to power cycle the modem. Any idea why? And why this only happens on the desktop and not the laptop?

Thanks in advance.
Connect your Desktop's MAC Address to the router and Insert your preferred DNS to your Local area connection TCP/IPv4(local area connection Protocol Version 4)
Are you connected with DHCP Via LAN with your Desktop Or Static Address Via LAN?
 

tron9901

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Dec 13, 2009
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Connect your Desktop's MAC Address to the router and Insert your preferred DNS to your Local area connection TCP/IPv4(local area connection Protocol Version 4)
Are you connected with DHCP Via LAN with your Desktop Or Static Address Via LAN?
Not sure what you mean by "connect your Desktop's MAC Address to the router." I no longer have a router.

And I'm using DHCP on both the Desktop and Laptop.
 

tron9901

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Dec 13, 2009
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This is normal. Your modem will lock onto the mac address of the connected device and won't let go until it's usually restarted.

What you can do in the meantime is get the comcast equipment for a month or two as it has a built in router so you don't have to keep switching cables like this.

If you plan to only use wired, you can probably pick up a cheap router that is gigabit and weak on the wifi side to use for a while.
But if it's because the modem locks onto the mac address of the connected device and won't let go, wouldn't the problem happen on both connected devices then? I've switched back and forth numerous times and the problem only happens on the desktop. The desktop is the only device which requires a modem restart.
 
You might find a clue in one of the logs in the modem but it does not mean a lot since you can't really set anything.

This is a known restriction on modems. I am somewhat surprised that it does not require reboots for all changes

Since you can't have them plugged in at the same time just duplicate the mac address of your laptop on your desktop machine. There should be a field that lets you key in the mac address in the nic settings. You will have to fix this before you try to use both machine on your new router.

If you look at the screens in the modem you will find almost nothing you can change. Modems even when you purchase them are configured and controlled by the ISP. They have all kinds of feature and fields only the ISP can see with their equipment.
 
Reactions: SamirD

tron9901

Distinguished
Dec 13, 2009
5
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18,515
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You might find a clue in one of the logs in the modem but it does not mean a lot since you can't really set anything.

This is a known restriction on modems. I am somewhat surprised that it does not require reboots for all changes

Since you can't have them plugged in at the same time just duplicate the mac address of your laptop on your desktop machine. There should be a field that lets you key in the mac address in the nic settings. You will have to fix this before you try to use both machine on your new router.

If you look at the screens in the modem you will find almost nothing you can change. Modems even when you purchase them are configured and controlled by the ISP. They have all kinds of feature and fields only the ISP can see with their equipment.
Took my laptop MAC address, duplicated it on my desktop and that fixed the problem! Thanks for the great suggestion!
 
But if it's because the modem locks onto the mac address of the connected device and won't let go, wouldn't the problem happen on both connected devices then? I've switched back and forth numerous times and the problem only happens on the desktop. The desktop is the only device which requires a modem restart.
It should with both, but this is something that the head end locks onto, not necessarily your modem. When you power cycle the modem, it disconnects from the head-end and reconnects, hence why this works.

You really just need some sort of router on the network, even if it is a basic one without wifi. The days when isps would hand out multiple dhcp IPs is over because otherwise your setup would work fine and even a small unmanaged switch would work.
 
Reactions: tron9901
Took my laptop MAC address, duplicated it on my desktop and that fixed the problem! Thanks for the great suggestion!
Glad this workaround fixed it for you. But the caveat is that you absolutely SHOULD NOT have both computers connected to any network at the same time as it will cause problems with a duplicate mac address. Be sure to change it back before you connect both again.
 

tron9901

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Dec 13, 2009
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Glad this workaround fixed it for you. But the caveat is that you absolutely SHOULD NOT have both computers connected to any network at the same time as it will cause problems with a duplicate mac address. Be sure to change it back before you connect both again.
Absolutely. Thanks again!
 
Reactions: SamirD

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