The total amount of U-height available in a 42U high server rack is 42 x 1U. This means that the server rack can accommodate a total equipment height of 42 or in server terms 42x 1U high server racks. This is for a fully populated server cabinet and in reality not all the slots are filled and for cooling and air flow purposes space may be left between the servers themselves and other devices in the cabinet such as an uninterruptible power supply or rack mounted fire suppression system. If spaces are left, this can lead front slots being vacant and these should be filled using blanking space to ensure efficient cooling.