Re: GUID's and Uniqueness.

froliol_at_yahoo.com (Louis Frolio) wrote in message news:<94c28610.0409140636.2b28f6bc_at_posting.google.com>...
> Greetings All, I have read many upon many articles here regarding GUID
> data types and uniqueness. There have been many opinions regarding
> the effectiveness of GUID's and when they should/should not be used.
> However, every article strongly implies, if it does not state it
> outright, that GUID's are always unique. My question is this, what
> happens if you have a database that uses GUID's and the NIC is changed
> out on the box? From what I understand the MAC address of the NIC is
> used as part of the algorithm to generate a GUID. If you change out
> the NIC after generating 1 billion GUID's do you run the chance of
> generating a duplicate GUID?
>
> I look forward to your insightfulness on this issue.
>
> Regards, Louis.

Since Ethernet uses a 48 bit address space, there are potentially 2^48 or 281,474,976,710,656 possible MAC addresses. You've generated 1 billion GUIDs with a single MAC address; I doubt changing a NIC would result in duplicate GUID values with over 200 trillion MAC addresses to possibly choose from. You could, possibly, change the MAC address for your currently installed NIC to that of the previous NIC, if you had the foresight to make note of it somewhere. Some software licenses are bound to a specific MAC address. Changing the MAC address in this way, unfortunately, is not permanent: after a reboot, it will revert to the MAC address physically stored in the card.

Under Linux, the MAC address of a Network Interface Card (NIC) can be changed using a command such as

ifconfig eth0 hw ether 00:01:02:03:04:05

or

ip link set eth0 address 00:01:02:03:04:05 (This needs to be done before network initialization.)

Under Windows XP, the MAC address can be changed in the Ethernet adapter's Properties menu, in the Advanced tab, as "MAC Address", "Locally Administered Address", or "Ethernet Address". The exact name depends on the Ethernet driver used; not all drivers support changing the MAC address in this way.

I hope this helps.

David Fitzjarrell Received on Tue Sep 14 2004 - 15:45:20 CDT