The biggest difference between Ethernet II and 802.3 is the fields of their Ethernet headers. Ethernet II is much more popular for reasons that I’ll make clear shortly. Its header fields are:
- Preamble: This offers synchronization since both sender and receiver interface cards are running with different system clocks.
- Start frame delimiter: This tells the Ethernet software where to start reading the frame.
- Destination address: The Media Access Control (MAC) address where this frame is supposed to go.
- Source address: The MAC address of the sending device.
- Type field: Sets the kind of packet that is in the data field. Its also called Ether Type.
- Data field: Carries the data application data plus networking overhead.
- Frame check sequence: The sending NIC does a calculation on the bit stream and puts the result in this field. The receiver of the frame then does the same calculation on the bit stream that it received and compares the two values. If the bit stream has been changed, the match will fail, and the frame will be throwaway.
There are three frame formats from the IEEE: IEEE 802.3, IEEE 802.3 with SNAP, and 802.3 with 802.2. Modern operating systems can send and receive any of these frame formats. The main difference between Version II and IEEE frames is that the Type field in Version II has been replaced with a 2-byte Length field in the IEEE formats.
To run TCP/IP over IEEE 802.3, the SNAP format has to be used. That requires 8 bytes of the data field to identify the kind of data the frame is carrying: three bytes for the Logical Link Control, three bytes for the SNAP header, and two bytes for the Protocol Type field. That means the data field shrinks from the standard range of 46 to 1500 bytes down to a range of 38 to 1492. This is the reason most network managers stay with Ethernet II.
The Ether Type field contains two bytes of hexadecimal code. This table translates the codes to some of the more commonly used codes:
|0x0800||Internet Protocol, Version 4 (IPv4)|
|0x0806||Address Resolution Protocol (ARP)|
|0x8035||Reverse Address Resolution Protocol (RARP)|
|0x8100||VLAN-tagged frame (IEEE 802.1Q)|
|0x814C||Simple Network Management Protocol (SNMP)|
|0x86DD||Internet Protocol, Version 6 (IPv6)|
|0x888E||EAP over LAN (IEEE 802.1X)|
|0x88E5||MAC security (IEEE 802.1AE)|
|0x88F7||Precision Time Protocol (IEEE 1588)|
Understanding Networking Fundamentals