First off, I did some research on the OP's device. I'm calling it a device, and not a "cable modem", because of what I found out.
My information comes from this page
No normal "Cable Modem" has four 10/100/1000 Gig Ports on it, along with 802.11n wireless technology.
This tells me that the OP has a "Combo" or "Gateway" device. They don't have a "standard cable modem". What I mean by "standard cable modem" would be one like a Motorola Surfboard SB5100.
Since the OP has not said where they "purchased" this "new Motorola SBG6580" from, anything we post here is just assumptions.
However, I will say that it is quite possible if the OP did purchase it from a friend, or online from Craigslist, ebay, or other like website, there is a good chance that Comcast has "blacklisted" the device. The reasons for that action by Comcast have already been listed by graysonf
I also agree that there is no way to change the MAC address on a standard cable modem. Modifications to the MAC address of a router are possible however. In fact, I remember back in the day when I first got cable internet (ISP was @home), I had to "clone" my MAC Address of my PC into my router, as the modem would not allow me to go online, as I had to register first using a PC without the router. Once I added the router (which, btw was a Netgear RP114), there was a place in the router to "clone" the MAC of a PC (or you could just type one in). In this way, that MAC was passed onto the cable modem, and all was well. I don't believe one has to do that anymore.
Could the MAC on the router side of the OP's device be modified? I believe so. But, that's not the reason the OP cannot use it.
If the OP does respond, and they tell us that they purchased it new from a brick and mortar store (Best Buy, Wal-Mart, etc), and shows us proof of such transaction, I think it would be time for the OP to return the device for a new one, as it would appear that the "new" item was not really new.
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