generate random locally administered MAC address

gistfile1.sh

# choose the leading octet:
echo -en '\x06' | xxd -b
# gives:
# 0000000: 00000110 
#    where these ↑↑ digits must be 10
# i.e. an octet with the the hex representation x2, x6, xA or xE

# then take the chosen octet and append a random number:
echo -n "06:" ; openssl rand -hex 5 | sed 's/\(..\)/\1:/g; s/.$//'

# if lots need generating, use the first as the initial, and increment the number so there's no collisions 
# (only a worry with amounts over about 500,000 — see the birthday problem)