When installing Xen servers one issue that arises is how to assign MAC addresses. The Wikipedia page about MAC addresses  shows that all addresses that have the second least significant bit of the most significant byte set to 1 are “locally administered”. In practice people just use addresses starting with 02: for this purpose although any number congruent to two mod four used in the first octet would give the same result. I prefer to use 02: because it’s best known and therefore casual observers will be more likely to realise what is happening.
Now if you have a Xen bridge that is private to one Dom0 (for communication between Xen DomU’s on the same host) or on a private network (a switch that connects servers owned by one organisation and not connected to machines owned by others) then it’s easy to just pick MAC addresses starting with 02: or 00:16:3e: (the range assigned to the Xen project). But if Xen servers run by other people are likely to be on the same network then there is a problem.
Currently I’m setting up some Xen servers that have public and private networks. The private network will either be a local bridge (that doesn’t permit sending data out any Ethernet ports) or a bridge to an Ethernet port that is connected to a private switch, for that I am using MAC addresses starting with 02:. As far as I am aware there is no issue with machine A having a particular MAC address on one VLAN while machine B has the same MAC address on another VLAN.
My strategy for dealing with the MAC addresses for the public network at the moment is to copy MAC addresses from machines that will never be in the same network. For example if I use the MAC addresses from Ethernet cards in a P3 desktop system running as a router in a small company in Australia then I can safely use them in a Xen server in a co-location center in the US (there’s no chance of someone taking the PCI ethernet cards from the machine in Australia and sending them to the US – and no-one sells servers that can use such cards anyway). Note that I only do this when I have root on the machine in question and where there is no doubt about who runs the machine, so there should not be any risk.
Of course if someone from the ISP analyses the MAC addresses on their network it will look like they have some very old machines in their server room. ;)
I wonder if there are any protocols that do anything nasty with MAC addresses. I know that IPv6 addresses can be based on the MAC address, but as long as the separate networks have separate IPv6 ranges that shouldn’t be a problem. I’m certainly not going to try bridging networks between Australia and the US!
Another possible way of solving this issue would be to have the people who run a server room assign and manage MAC addresses. One way of doing this would be to specify a mapping of IP addresses to MAC addresses, EG you could have the first two bytes be 02:00: and the next four be the same as the IPv4 address assigned to the DomU in question. In the vast majority of server rooms I’ve encountered the number of public IP addresses has been greater than or equal to the number of MAC addresses with the only exception being corporate server rooms where everything runs on private IP address space (but there’s nothing wrong with 02:00:0a: as the prefix for a MAC address).
I also wonder if anyone else is thinking about the potential for MAC collisions. I’ve got Xen servers in a couple of server rooms, I told the relevant people in writing of my precise plans (and was assigned extra IP addresses for all the DomUs) but never had anyone mention any scheme for assigning MAC addresses.