Ethernet frame types

There are several types of Ethernet framing, but over the years only two types have prevailed: IEEE 802.3 and Ethernet II. The main difference between the two is the use of the field that comes before the payload. Take a look at these two diagrams of the Ethernet fields for each type:

IEEE 802.3 Ethernet Header:

Ethernet II Header:

The 802.3 Ethernet header has a Length field while the Ethernet II header has an EtherType field. Both fields are two bytes in size. For more details about the Ethernet header, take a look at the Ethernet header.

The Length field of the 802.3 Ethernet header indicates the length or size of the payload (data) in bytes.

The EtherType field is used to identify the upper layer protocol (e.g. IP) that encapsulates the frame. For example, a value of 0x0800 indicates an IPv4 packet while a value of 0x0806 indicates an ARP frame. These values share the same numbering scheme as those found in the PTYPE field in the header of an ARP message.

Today, both of these formats can coexist on the same network. The method by which network devices know how to interpret each frame as one or the other is by the value found in the EtherType/Length field.

• If it is >= 1536 (0x0600) then it is an Ethernet II frame and that field is interpreted as an EtherType field.
• If it is <= 1500 it is an 802.3 frame and that field is interpreted as a Length field.
• Values between 1501 and 1535 are considered undefined and under normal circumstances should never be encountered.

Using this method of differentiation between the two, both standards can coexist on the same medium. However, Ethernet II is the most commonly used type.

Links

https://networklessons.com/cisco/ccna-200-301/introduction-to-ethernet#Data_Link_Layer