Draw a mac frame and elaborate its different fields justify the reason behind minimum and a maximum length of MAC frame.


The basic frame format which is required for all MAC implementation is defined in IEEE 802.3 standard. Though several optional formats are being used to extend the protocol’s basic capability.
Ethernet frame starts with Preamble and SFD, both works at the physical layer. Ethernet header contains both Source and Destination MAC address, after which the payload of the frame is present. The last field is CRC which is used to detect the error. Now, let’s study each field of basic frame format


IEEE 802.3 standard





  • PREAMBLE – Ethernet frame starts with 7-Bytes Preamble. This is a pattern of alternative 0’s and 1’s which indicates starting of the frame and allows sender and receiver to establish bit synchronisation.
  • Destination Address – This is a 6-Byte field that contains the MAC address of the machine for which data is destined.
  • Source Address – This is a 6-Byte field that contains the MAC address of the source machine. As Source Address is always an individual address (Unicast), the least significant bit of the first byte is always 0.
  • Length – Length is a 2-Byte field, which indicates the length of the entire Ethernet frame. This 16-bit field can hold the length value between 0 to 65534, but the length cannot be larger than 1500 because of some own limitations of Ethernet.
  • Data – This is the place where actual data is inserted, also known as Payload. Both IP header and data will be inserted here if Internet Protocol is used over Ethernet. The maximum data present may be as long as 1500 Bytes. In case data length is less than minimum length i.e. 46 bytes, then padding 0’s is added to meet the minimum possible length.
  • Cyclic Redundancy Check (CRC) – CRC is a 4 Byte field. This field contains a 32-bits hash code of data, which is generated over the Destination Address, Source Address, Length, and Data field.






DA [Destination MAC Address] : 6 bytes
SA [Source MAC Address] : 6 bytes
Type [0x8870 (Ethertype)] : 2 bytes
DSAP [802.2 Destination Service Access Point] : 1 byte
SSAP [802.2 Source Service Access Point] : 1 byte
Ctrl [802.2 Control Field] : 1 byte
Data [Protocol Data] : > 46 bytes
FCS [Frame Checksum] : 4 bytes

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