My dad implemented a MAC address block on my Xbox after a certain time so that I couldn't play online. I figured out a way around it.

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Due to a bug in OS X 10.5.6+, the built in mac-address spoofing function requires the following work around:

To determine your old mac-address:

ifconfig en1 | grep ether

Do not 'Turn Airport Off', instead deactivate your wireless card by:

Click 'Join Other Network...' and enter a fake SSID. Allow it to

attempt a connection and then go ahead and cancel the request. Your

card is now deactivated allowing us to proceed with the following

command:

sudo ifconfig en1 ether 00:11:22:33:44:55

Confirm using:

ifconfig en1 | grep ether


FOR XBOX

Before you start configuring, open up Terminal (in Applications -> Utilities) and type "ifconfig en0" (zero, not a capital-o). You're looking for the bit that says "inet xxx.xxx.xxx.xxx". In my case, the number I get here is 192.168.2.1 Second, note down the IP address of your wireless router, which in my case is 192.168.0.1


On your Mac

Connect your Mac to your wireless internet connection

Connect your Xbox 360 to your Mac via Ethernet

In System Preferences -> Sharing, enable Internet Sharing between your Airport and Ethernet connection


On your Xbox 360

Start up your Xbox and navigate to the Network settings menu

Under IP address, enter the number you got from the "ifconfig en0" command earlier on, but with the final number one place higher. In my case, I take the 192.168.2.1, and input 192.168.2.2 in the IP address box on my Xbox 360.

Under subnet, input 255.255.255.0

Under router/gateway, enter the number you got earlier from the "ifconfig en0" command. In my case, that's "192.168.2.1"

Move onto DNS. Enter the IP address of your wireless router (in my case, "192.168.0.1") in both the primary and secondary IP address fields

Leave PPPoE and Advanced settings blank (i.e. OFF and Automatic)


You should now be able to connect to Xbox Live via your internet sharing Mac