Editing EtherType (section)

== Overview ==
[[Image:EthernetFrame.jpg|thumb|center|700px|An Ethernet frame including the EtherType field. Each lower slot designates an octet; the EtherType is two octets long.]]

In modern implementations of Ethernet, the field within the Ethernet frame used to describe the EtherType can also be used to represent the size of the payload of the Ethernet Frame.  Historically, depending on the type of Ethernet framing that was in use on an Ethernet segment, both interpretations were simultaneously valid, leading to potential ambiguity.  [[Ethernet II framing]] considered these octets to represent EtherType while the original IEEE 802.3 framing considered these octets to represent the size of the payload in bytes.

In order to allow Ethernet II and IEEE 802.3 framing to be used on the same Ethernet segment, a unifying standard, IEEE 802.3x-1997, was introduced that required that EtherType values be greater than or equal to 1536.  That value was chosen because the maximum length ([[Maximum Transmission Unit|MTU]]) of the data field of an Ethernet 802.3 frame is 1500 bytes and 1536 is represented by the number 600 in the [[hexadecimal]] numeral system. Thus, values of 1500 and below for this field indicate that the field is used as the size of the payload of the Ethernet frame while values of 1536 and above indicate that the field is used to represent an EtherType.  The interpretation of values 1501–1535, inclusive, is undefined.<ref>IEEE Std 802.3-2005, 3.2.6</ref>

The end of a frame is signaled by a valid [[frame check sequence]] followed by loss of carrier or by a special symbol or sequence in the [[line code|line coding scheme]] for a particular [[Ethernet physical layer]], so the length of the frame does not always need to be encoded as a value in the Ethernet frame.  However, as the minimum payload of an Ethernet frame is 46 bytes, a protocol that uses EtherType must include its own length field if that is necessary for the recipient of the frame to determine the length of short packets (if allowed) for that protocol.