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EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same field is also used to indicate the size of some Ethernet frames.

EtherType is also used as the basis of 802.1Q VLAN tagging, encapsulating packets from VLANs for transmission multiplexed with other VLAN traffic over an Ethernet trunk.

EtherType was first defined by the Ethernet II framing standard and later adapted for the IEEE 802.3 standard. EtherType values are assigned by the IEEE Registration Authority.

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 (MTU) of the data field of an Ethernet 802.3 frame is 1500 bytes and 1536 (decimal) is 0x600 (hexadecimal).^{[citation needed]} 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.^[1]

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 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.

802.1Q VLAN tagging uses a 0x8100 EtherType value. The payload following includes a 2-byte tag control identifier (TCI) followed by an Ethernet frame beginning with a second (original) EtherType field for consumption by end stations. IEEE 802.1ad extends this tagging with further nested EtherType and TCI pairs.

The size of the payload of jumbo frames, typically up to ≈9000 octets long, collides with the range used by EtherType. Therefore, it cannot be used for indicating the length of such a frame in the old 802.2/802.3 formats that have a length field (i.e. LLC, SNAP and raw 802.3) in place of the EtherType field.

The IETF ISIS working group originally proposed^[2] to resolve this conflict by substituting 0x8870 as EtherType for frames exceeding 1500 octets in length. This was initially rejected by the IEEE’s 802.3 working group, as documented in an appendix^[3] to the IETF draft. However, about 15 years later, the concept was picked up by the 802.1 working group and incorporated into 802.1AC-2016^[4], but due to issues in confirming ownership of the 0x8870 value^[5], a new value (0xC9D1) was assigned. This value, had it gained any widespread use, would’ve led to interoperability issues and was relatively quickly rectified to use the original 0x8870 value in 802.1AC-2016/Cor 1-2018^[6]. 0xC9D1 thus became an historic oddity.

Effectively, 802.1AC-2016/Cor 1-2018 has thus retroactively standardized the practice used by router vendors in their IS-IS implementations (for IIH Hello packets padding and extended size LSPs).^[7]

Even though IEEE 802.3 does not currently permit jumbo frames with the commonly used limits in the 9000s of octets, it does permit payloads of up to 1982 octets^[8]. This means that for a long time, it was not possible to use the maximum standard frame size from IEEE 802.3 together with the encapsulations defined in IEEE 802.2.

With the advent of the IEEE 802 suite of standards, a Subnetwork Access Protocol (SNAP) header combined with an IEEE 802.2 LLC header is used to transmit the EtherType of a payload for IEEE 802 networks other than Ethernet, as well as for non-IEEE networks that use the IEEE 802.2 LLC header, such as FDDI. However, for Ethernet, Ethernet II framing is still used.

EtherTypes are assigned by the IEEE Registration Authority,^[9] which publishes them in list format.^[10] The Internet Assigned Numbers Authority has a separate list of some EtherType registrations, compiled from several sources, including the IEEE Registration Authority’s list and some other lists.^[11]

A beige background indicates protocols that have fallen into general disuse even in their intended fields, or have become completely historic.

• Port (computer networking)

• IEEE Registration Authority Tutorials Archived 2022-09-08 at the Wayback Machine
• IEEE EtherType Registration Authority Archived 2022-09-08 at the Wayback Machine
• IANA list of assigned EtherType values