Computer can't connect to the Internet through router

I stumbled across a problem where my computer would not connect to the Internet through the router.

First, I troubleshot the cables. I could connect to the Internet if I connected directly to the cable modem, however, as soon as the modem was connected to the router I would have no access. I was also able to access the router's administrative interface so the connection was good there as well.

The solution to my problem was that my Internet Service Provider (ISP) required the modem to be connected directly to the computer.  The router interfered with that. To fool the cable modem that you're connected to the computer, you must copy (clone) your computer's Mac address.  The Mac address is the computer's distinct identifier on the network.  To clone your computer's mac address, it is easily done through your router's admin interface. I have the Linksys WRT54GS and the cloning button is on the main admin screen.  Keep in mind that each router's admin interface is different so either consult your owner's manual on how to access this or else Google is your friend here.

Generally speaking, once you're in the router's administrative interface, you will typically find a button that says "clone mac address".  Click on this and you're ready to go.  If you don't have this button, you may have to find the Mac address on your computer and manually enter it into your router's Mac address field.  Again, try using Google to figure out how to access your Mac address.

Another issue could if that if you're using Windows, ensure that Window's firewall is disabled as most router's have a firewall built in.  Having that or another duplicate firewall solution active at the same time can cause problems.