At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are substantially different from one another with each speed having a distinct set of architecture design rules.

In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start Frame Delimiter (SFD) were combined into a single field, though the binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version, as both uses of the field were common throughout industry.

The Ethernet II Type field is incorporated into the current 802.3 frame definition. The receiving node must determine which higher-layer protocol is present in an incoming frame by examining the Length/Type field. If the two-octet value is equal to or greater than 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated.

Web Links Working with Ethernet Frames http://howto.lycos.com/lycos/step/ 1,,5+30+34556+34588+34592,00.html