I cannot draw a valid IP address.

[Frank12345]

I want to make sure I understand what you are suggesting here.  Are you saying that my cable modem has "locked on" to the MAC of my secondary computer, such that it is not recognizing either my main computer or my router?   If that is the case, I don't understand what you are suggesting as a fix.  I do not know how to open the settings page for the cable modem to tell it to recognize my other hardware.   I checked that page you linked me to for troubleshooting the modem, but that page has no useful information. 

Docs for Motorola SB5120.
http://support.cox.com/sdccommon/asp/contentredirect.asp?sprt_cid=be19f914-eb7a-4c96-a480-c4082dfb1791

Linksys Router Docs? We need the model number so we can search the docs and find out out how to change the MAC of the router so that it is the same as whatever PC gets into the modem.

As stated earlier, the modem does not care a bit about l your local IP address. It wants a MAC. It fell in love with your old computer and will not talk to anybody else. So you have to give the router that MAC
It has to have the MAC. Everything else is automatic once the modem sees the right MAC.

Once the router has the MAC, it knows what to do. It will hide the old PC from the modem and pretend to be the old PC. The modem will gush lots of TCP//IP and the router will do DHCP and you can have many PCs on the router.

Reset the old PC and the router will give it a local IP.

Sorry about my poor writing skills.

[Geek-9pm]

You have the general idea of how it works. However, getting the modem to recognize a different Mac address will not resolve the issue. The modem only wants to communicate with a single address. It does not do DHCP assignments. The router must do DHCP  IP assignments. The router must have the MAC address recognized by the modem. All traffic will be directed to the router, and the router will do its job to send stuff to the individual computers based on the IP addresses it assigned to the local computers.
So either you have to dig into them router documentation and find out how to assign MAC or else try another router.
As far as I know all lLinkSys routers have this ability. What is the model number of your router?

[Frank12345]

The Router is  a LinkSys NR041. 

But a few days ago I bought another router and hooked it up; the problem remained. 

You have the general idea of how it works. However, getting the modem to recognize a different Mac address will not resolve the issue. The modem only wants to communicate with a single address. It does not do DHCP assignments. The router must do DHCP  IP assignments. The router must have the MAC address recognized by the modem. All traffic will be directed to the router, and the router will do its job to send stuff to the individual computers based on the IP addresses it assigned to the local computers.
So either you have to dig into them router documentation and find out how to assign MAC or else try another router.
As far as I know all lLinkSys routers have this ability. What is the model number of your router?

[Geek-9pm]

Yes, your router does have an option to change the MAC address.
There was a firmware revision that made this a separate tab. But even if you have the earlier firmware it's still in there somewhere.

Here in this forum we are volunteers and do not get paid for this work. I personally do a lot of research on this and found that on many forums the common thought is that you have to use a MAC address. It is possible to get a connection without using a that feature, but you will not be able to share the connection with other computers and the connection is not reliable.

I'm going to give you a link to another forum where this has been directly answered in very clear terms. Both Comcast and Linksys but agree that you're supposed to clone the address of the primary computer. In this context primary computer means the one that Comcast used to set up your system.
Apparently there is some feature where the modem will pick up a new MAC address, but the documentation is not clear on how it does this. So the generally rule is you use the address that is working and go from there.
It appears that Comcast locks on to the first computer at fines during the installation process. There is some way to reset the modem, but that doesn't resolve the problem. You still have to change the MAC used by the router.
Muse over the link below and notice somebody  put  this question quite directly and the answer they get is simply you have to clone the MAC.

http://www.dslreports.com/forum/remark,15536499

For more information just Google: LinkSys Comcast MAC clone

[Frank12345]

I appreciate all the help.  I finally did today what I should have done weeks ago - I took the modem back to comcast and got another one.   And now my main computer is working.  I am thrilled.  But, I am worried that if I try to set up my network again, the same problem will occur.  I still don't understand how it happened in the first place.  I had used both computers for over a year without incident, and it was the secondary computer that was working and not the primary one that comcast set up.  And of course this problem happened out of nowhere without anyone making any changes or doing anything. 

Yes, your router does have an option to change the MAC address.
There was a firmware revision that made this a separate tab. But even if you have the earlier firmware it's still in there somewhere.

Here in this forum we are volunteers and do not get paid for this work. I personally do a lot of research on this and found that on many forums the common thought is that you have to use a MAC address. It is possible to get a connection without using a that feature, but you will not be able to share the connection with other computers and the connection is not reliable.

I'm going to give you a link to another forum where this has been directly answered in very clear terms. Both Comcast and Linksys but agree that you're supposed to clone the address of the primary computer. In this context primary computer means the one that Comcast used to set up your system.
Apparently there is some feature where the modem will pick up a new MAC address, but the documentation is not clear on how it does this. So the generally rule is you use the address that is working and go from there.
It appears that Comcast locks on to the first computer at fines during the installation process. There is some way to reset the modem, but that doesn't resolve the problem. You still have to change the MAC used by the router.
Muse over the link below and notice somebody  put  this question quite directly and the answer they get is simply you have to clone the MAC.

http://www.dslreports.com/forum/remark,15536499

For more information just Google: LinkSys Comcast MAC clone

[Frank12345]

After getting my main computer back online, I decided to try to set up my home network again, using the new cable modem but the same router as before. 

It failed.  With the router set up, neither computer would connect to the internet.  Much to my relief, upon disconnecting the router and connecting my main computer directly, it still worked. 

Does anyone have any ideas on what is going on with the router?  It works for my local network but won't allow either computer to connect to the internet.  I am still puzzled about this. 

[Geek-9pm]

The modem prefers a MAC address. Without a MC address, the connection will reset from time to time. With ta proper MAC address the connection reset will be transparent to the user.
The modem does dynamic Internet IP assignment. This is a feature of cable modem operation.
It will connect with one computer. Only one.
Just one time you must  give the MAC of the PC that was last on-line to the router. From then on the router will revive the Dynamic assignment from the modem,  whenever the modem wants to play tag.
The router, in turn, will assign local IP address for  all your computers.
Please rared your documentation.