EtherType is a two-

octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...

field in an Ethernet frame. It is used to indicate which

protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...

is encapsulated in the payload of the frame and is used at the receiving end by the

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

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 In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a network packet, data unit on an Ethernet link transports an Ethernet frame ...

standard and later adapted for the

IEEE 802.3 IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics ...

standard. EtherType values are assigned by the IEEE Registration Authority.

Overview

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.

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 is equivalent to the number 600 in the

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, he ...

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. The end of a frame is signaled by a valid

frame check sequence A frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol. Frames are used to send payload data from a source to a destination. Purpose All frames and the bits, bytes, and fields contained within ...

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.

VLAN tagging

802.1Q VLAN tagging uses an 0x8100 EtherType value. The payload following includes a 16-bit tag control identifier (TCI) followed by an Ethernet frame beginning with a second (original) EtherType field for consumption by

end station In networking jargon, a computer, phone, or internet of things device connected to a computer network is sometimes referred to as an end system or end station, because it sits at the edge of the network. The end user directly interacts with an ...

IEEE 802.1ad IEEE 802.1ad is an Ethernet networking standard. It is as an amendment to IEEE standard IEEE 802.1Q-1998 and was incorporated into the base 802.1Q standard in 2011. The technique specified by the standard is known as provider bridging and stack ...

extends this tagging with further nested EtherType and TCI pairs.

Jumbo frames

The size of the payload of non-standard jumbo frames, typically ~9000 Bytes long, falls within the range used by EtherType, and cannot be used for indicating the length of such a frame. The proposition to resolve this conflict was to substitute the special EtherType value 0x8870 when a length would otherwise be used. However, the proposition (its use case was bigger packets for IS-IS) was not accepted and it is defunct. The chair of IEEE 802.3 at the time, Geoff Thompson, responded to the draft outlining IEEE 802.3's official position and the reasons behind the position. The draft authors also responded to the chair's letter, but no subsequent answer from the IEEE 802.3 has been recorded. While defunct, this draft was implemented and is used in Cisco routers in their IS-IS implementation (for IIH Hello packets padding).

Use beyond Ethernet

With the advent of the

IEEE 802 IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LAN), personal area network (PAN), and metropolitan area networks (MAN). The IEEE 802 LAN/MAN Standards Committee (LMSC) maintai ...

suite of standards, a

Subnetwork Access Protocol The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by EtherT ...

(SNAP) header combined with an IEEE 802.2

LLC A limited liability company (LLC for short) is the US-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of ...

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.

Registration

EtherTypes are assigned by the IEEE Registration Authority. Not all well-known uses of EtherTypes are recorded in the IEEE list of EtherType values. For example, EtherType 0x0800 (used by

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...

) does not appear in the IEEE list. The

Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Inte ...

has a separate list of some EtherType registrations, compiled from several sources, including the IEEE Registration Authority's list and some other lists; that list includes 0800.

Values

References

External links

IEEE Registration Authority TutorialsIEEE EtherType Registration Authority
{{DEFAULTSORT:Ethertype Ethernet