Per Request, here's something I posted elsewhere that should be helpful.
Attn. Mods: If this needs to be recatigorized, then feel free to do so.
Ok, so I was THIS close to atomizing my brand new controller (just picked up the Halo4 bundle, yeah!) because I couldn't find useful information on MAC addresses for the XBOX 360 either on here or in what should have been a simple Google (sorry Bing ;-p) search and the console's own help interface left me at a loss. So now that I've found out how to do this, I can hopefully help some poor soul out there who's lost. This was Very important to my setup because I have my network security ghosted as best I can while still allowing gaming traffic without hassle. Yes, I now own all three of the current gen HD consoles (and this was the Hardest to setup network wise).
So once you've unboxed your console and followed all the fun stuff on screen (I told it to skip the network setup because I knew this was going to be a chore), you need to go follow this path through the Dashboard:
Settings : System : Network Settings : Advanced Options : Create Adhoc Network
You can choose either option in the last section, but since I'm just doing a quick test method this works just fine.
The menu asks you to 'Enter SSID'. I simply entered TEST as I'm just creating a dummy connection to get information.
Click 'Done' as you dont' need to configure anything!
The system will back you out automatically and have the 'TEST' connection as its focus.
Go ahead and hit the 'A' button to select it and hit 'A' again when it say to 'Configure the Network'.
You should now see something with two tabs of settings: 'Basic' and 'Additional'. You need to scroll right to change to the 'Additional Settings' tab.
Now this is where I got hung up before. I didn't realize the various things you can scroll through in these tabs were able to be edited!
Under the 'Additional Settings' tab, highlight 'Advanced Settings' and hit the 'A' button to select it.
You will now see a single (improperly formatted) MAC address. It will look something like: AB1C23D456EF. Make note of this for the rest of your setup.
When you enter this information into your router you will need to change that to something like this: AB:1C:23:D4:56:EF.
You're done with the 'TEST' connection now so hit the 'B' button until you're back to the 'Available Network' screen and hit 'Y' to remove the 'TEST' connection.
Normally, a device has a separate MAC address for each network connection (One for Wireless, One for Wired, etc), but for some reason the XBOX either uses the same address or doesn't show the WIRED MAC address when there is no ethernet cable attached. I'll let the Forum Mods clarify that point as I have no reason to test this.
Hopefully this has made somebody's day and if there is any information that needs to be clarified, then please post a meaningful reply.
I'd say 'see you out there', but I'm 99% sure that I'll not be using XBL. I'm just not that type of gamer.