For omitting certain devices (by mac address) from DHCP assignment:

server = 8.8.8.8
server = 8.8.4.4
dhcp-host=00:50:56:*:*:*,set:blocked
dhcp-mac=set:blocked,00:50:56:*:*:*
dhcp-ignore=tag:blocked

Used to prevent VMWare provisoned devices from picking up an address outside their lab.