An Ethernet switch includes 12-Volt and 48-Volt power sourcing modules, system software, Ethernet interface modules and optional power over Ethernet (PoE) modules. The Ethernet interface modules are motherboards that include the circuitry required to implement a non-PoE system. The PoE modules are daughter boards that include the circuitry required to supply powered devices in a PoE subsystem. A PoE module may be connected to a corresponding Ethernet interface module. During start up, all of the Ethernet interface modules are first powered up in response to the 12-Volt power sourcing module. If the system software subsequently determines that the 48-Volt power sourcing module is operational, then (and only then) the system software attempts to detect the presence of any PoE modules. Upon detecting one or more PoE modules, the PoE modules are initialized and configured, thereby enabling PoE operation.