# File lib/deltacloud/drivers/condor/ip_agents/confserver.rb, line 47
    def find_free_mac
      addr_hash = {}
      DefaultExecutor::new do |executor|

        # Make an CondorAddress hash to speed up the inner loop.
        CondorAddresses.each do |address|
          addr_hash[address.mac] = address.ip
        end

        executor.instances.each do |instance|
          instance.public_CondorAddresses.each do |public_CondorAddress|
            if addr_hash.key?(public_CondorAddress.mac)
              addr_hash.delete(public_CondorAddress.mac)
            end
          end
        end
      end

      raise "No available MACs to assign to instance." if addr_hash.empty?

      return addr_hash.keys.first
    end