The Enterprise Chassis supports up to a total of ten hot-swap fan modules: two 40 mm fan modules and eight 80 mm fan modules. The two 40 mm fan modules distribute airflow to the I/O modules and chassis management modules. Both of these fan modules ship with the chassis. The 80 mm fan modules distribute airflow to the compute nodes through the chassis from front to rear. Each 80 mm fan module actually contains two 80 mm fan modules, back to back at each end of the module, which are counter-rotating. The following figure shows the 80 mm fan module. Four 80 mm fan modules are installed standard in chassis model 8721-A1x and CTO orders. The maximum number of 80 mm fan modules that can be installed is eight.