http://www.simplehelp.net/2006/07/11/cable-modem-troubleshooting-ambit-u10c018/

Many cable ISPs lock onto the ethernet MAC address of the connecting PC and if you switch to another the new PC won't work. Sometimes you have to power cycle the MODEM and then reboot the PC. Sometimes you have to wait up to 24 hours for your ISP to let the new PC on.

These rules can't be known by me since every cable ISP is different. I do have my solution which sidesteps this. It's called a router which has it's own MAC address so the issue is nullified.

Sorry I decline to define what a MAC address is. Just google that.

Bob