Ethernet is the dominant LAN technology. It is a family of networking technologies that are defined in the IEEE 802.2 and 802.3 standards.
Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. Ethernet is the most widely used LAN technology and supports data bandwidths of 10 Mbps, 100 Mbps, 1 Gbps (1,000 Mbps), or 10 Gbps (10,000 Mbps).
The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet. However, the methods for detecting and placing data on the media vary with different implementations.
Ethernet provides unacknowledged connectionless service over a shared media using CSMA/CD as the media access methods. Shared media requires that the Ethernet frame header use a data link layer address to identify the source and destination nodes. As with most LAN protocols, this address is referred to as the MAC address of the node. An Ethernet MAC address is 48 bits and is generally represented in hexadecimal format.
The figure shows the many fields of the Ethernet frame. At the data link layer, the frame structure is nearly identical for all speeds of Ethernet. However, at the physical layer, different versions of Ethernet place the bits onto the media differently. Ethernet is discussed in more detail in the next chapter.