iMac can only get online through router not direct to cable modem.

Hi

Yes. Typically the WAN IP address is assigned dynamically and is 'tied' to the ethernet hardware address (MAC address) of whatever the cable modem is connected to. If you disconnect from the cable modem and attach another device to it - another mac for example - then that newly attached mac won't be able to access the internet because its MAC address is different. MAC addresses are unique and they look something like this:

00:10:635:90:z1:9f

You can find the MAC address of your cable modem by looking for it on the modem itself. There should be a small label listing it. For the mac you use Network Utility to find the mac's MAC address or Apple System Profiler > Network or the command line.

To get the cable modem to 'see' the new device you reset the cable modem (simply switch it off), leave it for 10 minutes (go and make a cup of coffee) come back, switch it back on and connect the device. You should now be on the internet.

You can verify the connection by having the Network Preferences Pane open and set to DHCP and you should see an address similar to this: 81.124.64.214 appearing in the IP address field as well as 255.255.255.255 appearing in the sub-net mask field and another IP address in the default gateway field that will look similar to the first IP address.

Tony