Link layer functionality is divided between the network interface card/driver combination and the low-level protocol stack driver. The network card/driver combination filters are based on the destination media access control (MAC) address of each frame.
Normally, the hardware filters out all incoming frames except those containing one of the following destination addresses:
Because this first filtering decision is made by the hardware, the NIC discards any frames that do not meet the filter criteria without incurring any CPU processing. All frames (including broadcasts) that pass the hardware filter and Frame Check Sequence (FCS) validation are then passed up to the NIC driver through a hardware interrupt.2 The NIC driver is software that runs on the computer, so any frames that make it this far require some CPU time to process. The NIC driver brings the frame into system memory from the interface card. Then the frame is indicated (passed up) to the appropriate bound transport driver(s). The NDIS 5.1 specification provides more detail on this process.
Frames are passed up to all bound transport drivers in the order that they are bound.
As a packet traverses a network or series of networks, the source MAC address is always that of the NIC that placed it on the media, and the destination MAC address is that of the NIC that is intended to pull it off the media. This means that, in a routed network, the source and destination MAC address changes with each hop through a network-layer device (router or Layer 3 switch).
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