You can find the MAC address for LAN1/eth0 (not the BMC MAC) via the SuperMicro IPMI interface by running the following command:
$ ipmitool -U $IPMI_USER -P $IPMI_PASS -H $IPMI_HOST raw 0x30 0x21 | tail -c 18
The eth0 MAC address will be output in this format:
00 25 90 f0 be ef
I found this discussion, but it took me about an hour to implement it. To save others time, here are the most important lessons:
On a Supermicro motherboard, if you know the MAC address of LAN 1, just add 1 to the last byte of the address. See http://www.supermicro.com/support/faqs/faq.cfm?faq=14914. (I know, boxrick just referenced this page, but knowing the MAC addresses are sequential might save you the trouble of looking this up.)
On an Apple Mac, use Homebrew to install ipmitool. Open a terminal window, and enter "brew install ipmitool". If you don't have Homebrew installed yet, use this command first:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" < /dev/null 2> /dev/null
See macappstore.org/ipmitool/.