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) interprets the 2-octet field following the destination and source addresses as an EtherType that immediately identifies an upper-layer protocol. The original version of IEEE 802.3 changed that field to a length field, with an IEEE 802.2 header expected to follow that field; in the 802.3x-1997 standard, and later versions of the 802.3 standard, both types of framing are specified.

For example, with Ethernet v2 framing, an EtherType value of 0x0800 signals that an IP datagram is encapsulated. Likewise, an EtherType of 0x0806 indicates an ARP frame.

In order to allow both packets using Ethernet v2 framing and packets using the original version of 802.3 framing to be used on the same Ethernet segment, 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 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.