Ethernet: Addressing

There are three types of Ethernet MAC Addresses:

  • Unicast – An address that represents a single LAN interface.  The I/G bit is set to 0.
  • Broadcast – An address that represents ‘all devices on a LAN’ Always a value of hex FFFFFFFFFFFF
  • Multicast – A MAC address that implies some subset of all devices on a LAN.  The I/G is set to 1.

When an Ethernet NIC is aware of the specific MAC address it intends to send frames to, it places the specific Unicast MAC address of the device in the destination field of the header and forwards the frame to that single device only.

When an Ethernet NIC needs to send traffic to the Broadcast address (FFFF.FFFF.FFFF) that Frame will be forwarded to ALL devices on the same LAN/VLAN.

Multicast frames are used to communicate with a dynamic subset of devices on a LAN. For example if only 3 out of 100 network devices on a LAN want to participate and watch a video stream.  These devices interested in participating can do so by listening for frames sent to the specific multicast MAC address.  Other devices that may receive the frame will drop the frame if they are not participating in the group.

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