If you haven't already, I recommend reading the Home Networking Help Page to get a basic understanding of networking before reading this page. There is some duplication but it's worth reading.

A Cable ISP assigns the external IP address for a Cable Modem based on the MAC address of the first device connected to the Modem. The Cable Modem passes this external IP address to this same first device. I have never found it to be otherwise with Cable Modems. This is done to prevent people from cloning or spoofing the MAC address of the Modem itself to gain unregistered access to the cable network and also to pass along all of the proper IP's such as DNS addresses. The MAC address of the Modem is used to identify the specific Modem to verify the users subscription which then allows or denys access to the ISP's network but the external IP address is assigned to the MAC address of the first device connected to the Cable Modem.

To get a new IP address from your Cable ISP, the MAC address of the first device must change either through cloning some other address to the device or through physically changing the device. For the ISP to discover the new device or MAC address and assign a new IP address, The old MAC address must be cleared from the Cable Modem. To do this, a cold boot of the Cable Modem is required. That means the Modem MUST be powered off. If any of the Modem lights are still on, the Modem is NOT powered off. Some Modems have a battery backup built in. The battery must be removed to do a cold boot.

I recommend doing a cold boot of the entire network. To do this, all equipment connected to the network by ethernet must be powered off. This includes the Modem, Router, computers, VoIP boxes, gaming boxes, print servers and any other devices connected to the internal network by ethernet cable. The devices only need to be powered off, not physically disconnected from the network.

If you are cloning the MAC address of the Router, do that before powering the device off. Now make all of the physical connection changes while the power is off.

To start the network, first start ONLY the Cable Modem and allow it to boot. Usually about one minute. Next start the Router and allow it to boot. Usually about 30 seconds. The rest of the equipment can be started in any order.

You should now be connected to the internet with a new external IP address.

Happy Surfing
Shnerdly