So if you have cable Internet service, such as Comcast, and change computers or routers and notice you can’t get connected to the internet anymore, read on.
Cable ISPs such as Comcast register the modem by MAC address. This MAC address is connected to your account, and if a modem is connected to your line that has a MAC address which is not connected to your account, you will not have connectivity. To solve this, simply call your cable ISP and give them the modem’s MAC address and ask that they make sure it’s added to your account.
Next, the modem identifies the device behind the modem by MAC address. If you suddenly swap this device, the modem may choose not to speak with it until it is either power-cycled (or in some cases completely reset).
A fairly straightforward way is to connect a computer directly to the modem, attempt to go to a website (which will cause your computer to have it’s MAC address registered), then connect a router and use the ‘MAC Address Emulation’ feature to clone the PCs MAC address. That should get you up again.