The standards body IEEE has a list of something that's called OUI. If this is a new TLA (Three Letter Acronym) for you, it expands to Organizational Unique Identifier.

An OUI is actually one or more ranges of MAC adresses assigned to an organization that's issuing network cards (real or virtual).

And wouldn't you know, XenSource, recently acquired by Citrix, has it's own OUI.

This prompted a new release of EasyMAC, the MAC address generator.

The new features are:

• Generation of valid XenSource MAC addresses.
• A new option to output only the MAC address, without any extra text.

Head over to the download page for more info and downloads.