Ethernet Frame Format:

Ethernet Frame Field Descriptions

• Preamble---preamble is used to inform the receiving stations that a frame is coming, and provide a means to synchronize the frame-reception portions of receivers physical layers. The preamble consists of 7 octets of alternating ones & zeros followed by a single octet, called the start of frame delimiter (SOF) which consists of 6 alternating bits ending in two one-bit values. Note: this is rarely shown in examples.

• Destination address---MAC address that identifies which node(s) should receive the frame.

• Source address---MAC address that identifies the sending node of the frame.

• Length or Type---indicates either the length of the data portion of the frame in octets, or the type of the data contained within the data portion of the frame. If the value is <= 1500 (x05DC), the value is a length value. If the value is >=1536 (x0600), the value is a type value. Example type values are:

  	x0800: IPv4
  	x0806: ARP
  	x8137: IPX
  	x86DD: IPv6
  	x9000: Loopback
  

  (see http://standards.ieee.org/develop/regauth/ethertype/eth.txt or http://www.iana.org/assignments/ethernet-numbers )

• Data---a sequence of n octets, where n must be <= 1500. If the length of n is < 46, data is added to “pad” the length of the data to 46 bytes.

• Frame Check Sequence---specifically for 802.3, this is a 32-bit Cyclic Redundancy Check (CRC-32).