Ethernet is the most widely used LAN technology today. 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. For the Layer 2 protocols, as with all 802 IEEE standards, Ethernet relies on the two separate sublayers of the data link layer to operate, the Logical Link Control (LLC) and the MAC sublayers.
At the data link layer, the frame structure is nearly identical for all speeds of Ethernet. The Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent.
There are two styles of Ethernet framing: IEEE 802.3 Ethernet standard and the DIX Ethernet standard which is now referred to Ethernet II. The most significant difference between the two standards is the addition of a Start Frame Delimiter (SFD) and the change of the Type field to a Length field in the 802.3. Ethernet II is the Ethernet frame format used in TCP/IP networks. As an implementation of the IEEE 802.2/3 standards, the Ethernet frame provides MAC addressing and error checking.
The Layer 2 addressing provided by Ethernet supports unicast, multicast, and broadcast communications. Ethernet uses the Address Resolution Protocol to determine the MAC addresses of destinations and map them against known network layer addresses.
Each node on an IP network has both a MAC address and an IP address. The node must use its own MAC and IP addresses in the source fields and must provide both a MAC address and an IP address for the destination. While the IP address of the destination will be provided by a higher OSI layer, the sending node must find the MAC address of the destination for a given Ethernet link. This is the purpose of ARP.
ARP relies on certain types of Ethernet broadcast messages and Ethernet unicast messages, called ARP requests and ARP replies. The ARP protocol resolves IPv4 addresses to MAC addresses and maintains a table of mappings.
On most Ethernet networks, end devices are typically connected, in a point-to-point basis, to a Layer 2 LAN switch. A Layer 2 LAN switch performs switching and filtering based only on the OSI data link layer (Layer 2) MAC address. A Layer 2 switch builds a MAC address table that it uses to make forwarding decisions. Layer 2 switches depend on routers to pass data between independent IP subnetworks.
Layer 3 switches are also capable of performing Layer 3 routing functions, reducing the need for dedicated routers on a LAN. Because Layer 3 switches have specialized switching hardware, they can typically route data as quickly as they can switch.