Ethernet, MAC Addressing & Switch Forwarding

An Ethernet MAC address is a 48-bit physical address expressed as 12 hexadecimal digits (6 bytes).

MAC Address Structure

IPv4 multicast MAC addresses begin with the reserved prefix 01-00-5E (e.g., 01-00-5E-00-00-03). A broadcast MAC address is FF-FF-FF-FF-FF-FF.

Multicast MAC Addressing

The Frame Check Sequence (FCS) field in the frame trailer uses a Cyclic Redundancy Check (CRC) value to determine whether errors occurred during transmission.

Error Detection (FCS)

Receives the entire frame and performs a CRC error check before forwarding. Frames containing errors are discarded.

Store-and-Forward Switching

Offers the lowest latency by forwarding the frame immediately after reading the destination MAC address, without CRC validation.

Cut-Through (Fast-Forward) Switching

A compromise that checks the first 64 bytes for collision fragments before forwarding.

Fragment-Free Switching

Enables a switch interface to automatically detect and configure itself to use either a straight-through or crossover Ethernet cable.

Auto-MDIX Feature

Switches inspect the destination MAC address in the frame header to forward traffic onward through the correct egress port

Layer 2 Forwarding Decisions