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Connected Cable modem only works on 1 of 5 PC's

Last response: in Networking
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October 3, 2013 6:49:25 AM

I have a Thomson DCM475 cable modem. I have it connected Cat6 to my main PC. Works perfect. Remove that cable from the back of that Win8(64) PC and plug it into any of my other 2 Win 8(64) laptops or with a new cat6 attach to my Storage PC or My HTPC and I have nothing. IPconfig would not ping a server on anything except the my main PC. I am frustrated as I have tried so many different scenarios and the only one that works is my main PC. Can the port be bad on the modem, although it is new? I have also tried connecting 2 different routers to the modem and neither one can get an IP from the modem.
October 3, 2013 6:53:48 AM

In between connecting devices, you probably have to power cycle the modem. It is probably looking for the MAC address of the device that was plugged in previously.
October 3, 2013 6:58:19 AM

Did you reset or power-cycle your modem each time you tried to attach it to a different PC?

Plain DOCSIS modem have a MAC address table tied to DHCP leases and most cablecos configure the modem to accept only one MAC address at a time which means only the first device to make a DHCP request will get a working address. If you want to clear the modem's table without having to wait for the DHCP lease to expire, you have to reset the modem whenever you change the MAC of whatever equipment you attach to it.

The simplest way to avoid having to do this would be to get a broadband router. Good models start around $80.
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October 3, 2013 7:53:09 AM

Please explain a bit more. I just switched from Broadband DSL to cable and purchased a Cable DOCSIS 3.0 recommended by the ISP provider. I tried USAFRet suggestion and reset the modem and that didn't work as such. But his suggestion made me think to try cloning the original PC Mac address into the router and supplying the DNS gateway. Viola. I sure don't know how this will affect future ISP IP changes or power outages. I want to be able to count on the service returning without all this hassel as I use the router for Telephone and DVR security remote monitoring and am not often physically present at modem location and the rest of the people present know nothing about setting this shit back up. Thanks USAFRet

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October 3, 2013 7:55:58 AM
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Mossey said:
Please explain a bit more. I just switched from Broadband DSL to cable and purchased a Cable DOCSIS 3.0 recommended by the ISP provider. I tried USAFRet suggestion and reset the modem and that didn't work as such. But his suggestion made me think to try cloning the original PC Mac address into the router and supplying the DNS gateway. Viola. I sure don't know ho this will affect future ISP IP changes or power outages. I want to be able to count on the service returning without all this hassel as I use the router for Telephone and DVR security remote monitoring and am not often physically present at modem location and the rest of the people present know nothing about setting this shit back up. Thanks USAFRet


Get a router and put it inbetween the modem and your devices. The modem will serve up the IP address to the router, and the router in turn gives out internal IP addresses.

Modem -> router -> devices.
If you have more than 4 devices, then Modem -> router -> switch -> devices.
October 3, 2013 8:30:41 AM

Or get a modem thats wireless. That makes it really easy.
October 3, 2013 8:33:43 AM

jitpublisher said:
Or get a modem thats wireless. That makes it really easy.


No, it still needs a router. The modem serves up one and only one IP address, to the first device that connects to it.
There are a lot of modem+router combinations, but there is still a router in there.
October 3, 2013 9:08:12 AM

Yeah, I tbought most wireless modems were routers too?
October 3, 2013 9:41:13 AM

Just to let you guys know that over the last 4 days I have been in touch with technical support with my ISP, Western Digital(Router) and several other forums. No one helped me with my problem like I was helped here. No one had any remotely close suggestions. It was always equipment screw-ups even though I had multiple pieces of hardware with same issues.
May be you can let me know if now that I have clone my PC address to the router, will that remain cloned after a power outage?

Again Thanks
October 3, 2013 9:47:13 AM

Mossey said:
Just to let you guys know that over the last 4 days I have been in touch with technical support with my ISP, Western Digital(Router) and several other forums. No one helped me with my problem like I was helped here. No one had any remotely close suggestions. It was always equipment screw-ups even though I had multiple pieces of hardware with same issues.
May be you can let me know if now that I have clone my PC address to the router, will that remain cloned after a power outage?

Again Thanks


Well...they are just reading off their script, and want to get you off the phone as soon as possible. Blame someone else's hardware or software.
We out here have no baggage like that.

But in truth, you should not have to clone the MAC address. Given sufficient reboot time for the modem, it should just pick up on whatever new device connects to it.
I'm not sure about the power outage/cloning thing. I've never, ever had to clone the MAC to get something to connect.
October 3, 2013 10:14:51 AM

USAFRet said:
Mossey said:
Just to let you guys know that over the last 4 days I have been in touch with technical support with my ISP, Western Digital(Router) and several other forums. No one helped me with my problem like I was helped here. No one had any remotely close suggestions. It was always equipment screw-ups even though I had multiple pieces of hardware with same issues.
May be you can let me know if now that I have clone my PC address to the router, will that remain cloned after a power outage?

Again Thanks


Well...they are just reading off their script, and want to get you off the phone as soon as possible. Blame someone else's hardware or software.


SPOT FRIGGEN ON USAFRet!!!
October 3, 2013 10:35:32 AM

Just an update! I powered off the router for 15 minutes to see if the cloning PC held up and it does. I was going to dick with it some more and reset everything to see if I could get service on the router IP, but decided to leave it as is, now if I ever have an issue I should be able to connect the PC directly to the modem.
!