Ethernet II framing

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Ethernet v2 framing, also known as DIX Ethernet (named after the major participants in the framing of the protocol: Digital Equipment Corporation, Intel, Xerox) differs from the original framing used in IEEE 802.3 by its interpretation of the 2-octet field following the destination and source addresses. With Ethernet v2 framing, that field is treated as an EtherType and immediately identifies an upper-layer protocol. For example, an EtherType value of 0x0800 signals that an IP datagram is encapsulated. Likewise, an EtherType of 0x0806 indicates an ARP frame. With the original framing used in IEEE 802.3, that field is treated as a packet length, and an IEEE 802.2 header is expected to follow the length field.

EtherType values must be greater than 0x0600; that value was chosen because Ethernet 802.3 frames aren't supposed to exceed 1500 bytes, so if the field's value is greater than 0x600, the frame must be an Ethernet v2 frame, with that field being a type field, and if it's less than or equal to 0x0600, it must be an original-style IEEE 802.3 frame, with that field being a length field. Versions of IEEE 802.3 starting with 802.3x-1997 support use of that field either as a type field or a length field, depending on its value.