1. A method for responding to a request to transfer an outgoing data frame from a virtual computer system to a computer network, the outgoing data frame comprising at least data to be transmitted and at least one of a layer 2 and layer 3 destination address, the virtual computer system comprising a host platform and a plurality of virtual machines (VMs) executing as guests on the host platform via virtualization software, the virtualization software comprising one or more layers of software interfacing between the VMs and the host platform, the virtual computer system further comprising a plurality of physical network interface cards (NICs), wherein each VM includes at least one virtual NIC, the method comprising the steps of: obtaining access by a NIC manager to the outgoing data frame, the outgoing data frame being provided by a particular one of the VMs via the virtual NIC of the particular VM, the NIC manager being a component of the virtualization software and being enabled to access data frames provided by any of the virtual NICs of the plurality of VMs; receiving, in the NIC manager, NIC management information related to one or more of the plurality of physical NICs; receiving, in the NIC manager, VM-specific information related to one or more of the VMs in the virtual computer system; and based on the NIC management information and the VM-specific information, selecting a physical NIC from the plurality of physical NICs and transferring the outgoing data frame to the computer network over the selected physical NIC, such that the outgoing data frame is generated by one of a plurality of VMs and is transferred to the computer network via the selected physical NIC of the plurality of physical NICs.