EthernetFrame

An EthernetFrame is the data-link layer protocol data unit used on Ethernet networks and defined by the IEEE 802.3 standard. It encapsulates higher-layer data for transmission over a shared medium and is processed by network interface cards and switches at the edge of the data link layer.

The typical Ethernet frame consists of a header, a payload, and a trailer. The header contains the destination MAC address (6 bytes), the source MAC address (6 bytes), and a 2-byte EtherType or Length field. The payload carries the upper-layer data and may also include optional headers such as an LLC or a VLAN tag. The trailer contains the Frame Check Sequence (CRC), a 4-byte value used to detect transmission errors. In addition to the frame itself, the transmission on the wire is preceded by a 7-byte preamble and a 1-byte start-of-frame delimiter, and is followed by an interframe gap, which helps separate consecutive frames.

Ethernet frames come in two framing interpretations: Ethernet II and the original IEEE 802.3 frame. In Ethernet II, the 2-byte EtherType field indicates the protocol of the payload (for example, IPv4 is 0x0800). In the original 802.3 framing, the field is a length indicator, and the payload may begin with an LLC header. VLAN tagging (IEEE 802.1Q) inserts a 4-byte tag after the source MAC, allowing multiple VLANs on a single link.

Size limits and addressing: the minimum payload is 46 bytes (to meet a 64-byte minimum frame size when header and trailer are included), and the maximum payload is 1500 bytes in standard Ethernet; total frame size is typically up to 1518 bytes, with larger “jumbo” frames supported in some networks. Ethernet frames can be unicast, multicast, or broadcast based on MAC addresses.