This may be a simple question, but I can't figure out the answer.
I have a DNS server that was using auto-configure to get an IPv6 address; that address ended in 41bd:d9bf:9b66:9b95
I recently converted that physical server to a virtual server, but since everyone was using the old address for DNS lookups, I assigned that same address to the "new" virtual server. (The 41bd:d9bf:9b66:9b95 address)
So far, so good.
I then needed to re-use that old physical machine; I was careful to plug the network cable into the second ethernet port, figuring if I used the first ethernet port I would get the 41bd:d9bf:9b66:9b95 address from last time.
My curiosity got the best of me and I decided to see what would happen (figuring I would break DNS momentarily while packets were routed to the wrong place) Much to my surprise, the new IPv6 address wasn't even close to being the same.
My question is, if the MAC address on the network card didn't change, shouldn't I have received the same IPv6 address when I re-installed the OS?
Your system used DAD (duplicate address detection). It saw that the address was already in use, and chose a different one.
So if I were to unplug the virtual server, would the physical server go back to the old address if i were to reinstall?
Probably, yes, if its not using privacy extensions.
Addresses generated from a MAC address doesn't look like the one in your example. The most recognizable part of an IPv6 address generated from a MAC address is the ff:fe inserted in the middle of the MAC address to generate the lower 64 bits of the IPv6 address. Also, one bit is changed from a zero to a one, and that one remains zero in your example. So I guess it was configured in some other way.