Ethernet Networks

  Ethernet evolved from researchers at Xerox. The term Ethernet was derived from the "mythical substance once assumed to permit light to travel through space." The original specification for Ethernet was established in September 1980, with a revised Ethernet II completed in November 1982. Ethernet II and the IEEE 802.3 specification are practically compatible.  
  Ethernet uses a method called carrier sensing for transmitting data over a network. This method allows multiple nodes (computers) to access the network in an organized manner for data flow. Sometimes simultaneous access occurs, resulting in a possible network collision. If this happens, each node must attempt to resend its data with a slightly modified time interval. A nominal amount of collision occurs on any network. Any properly designed network will keep the collision factor down to a minimum.  
  An Ethernet data frame consists of six sections: 1. The preamble (8 bytes in size), 2. The destination address (6 bytes in size), 3. The source address (6 bytes in size), 4. The type (2 bytes in size), 5. The data (46 - 1500 bytes in size), and the FCS (4 bytes in size). Sometimes the bytes of data in a frame are referred to octets. The preamble indicates that a frame is beginning. The destination address section is just that; it tells the network where the frame is going. The same applies for the source address; it tell where the frame came from. The type section indicates the data type (X.25, IP, ARP, etc.). The data section is the area that actually contains a bit of data that is being transferred from one node to another. The last section is the FCS, or frame check sequence. It indicates any errors that have occurred during the transferring of the frame.  
  The node address, also known as the MAC address, is the physical address that is permanently assigned to each individual node (in this case, the Ethernet card). It is 48 bits in size, divided into 6 8-bit sections. The first three sections determine the manufacturer of the card. The last three sections are used by the manufacturer themselves. There are no two MAC address alike for any Ethernet adapter on the market.  

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