When I try connecting my iMac G4 to my cable modem with a working ethernet cable, I cannot do so. Network Diagnostics gives me all reds, etc. The internet works fine with my MacBook.
Did you power off the cable modem when changing the Ethernet connection? Most cable modems require that you power off the cable modem when switching the Ethernet connection. When the modem is powered up it detects the new device and talks to that device.
Some cable modems have a reset button on the back to do this reset.
Some cable modems have a small internal battery which must be removed to do the reset.
Some cable modems must be turned off for several hours or overnight to reset the equipment back at the ISP.
I turned off the cable modem, unplugged it (which is usually either unnecessary or sufficient), and left it off for a day or two. Still nothing.
Also, in general, the computer 'sees' the ethernet connection, but can't connect to the internet. Now, it does not see the connection at all (but works perfectly with my MacBook).
So, that issue resolved itself. For a while, the iMac was connecting to the internet.
Then I came along, and plugged in my laptop. Now, the iMac won't connect.
At first, in network diagnostics, I got green for ethernet and yellow for network settings; the rest were red. Cablevision was contacted; they said the problem must be with the iMac not requesting an IP address, and that we should call Apple (something that, for obvious reasons, we can't do).
I manually put in the IP, router, DNS servers, etc. Now I get three greens and two reds in diagnostics. The computer itself seems rather unaware that it's not connected to the internet.
