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10: Ethernet - Ethernet II (version 2, or DIX) frame - source & destination MAC addresses fields part 11




Ethernet II (DIX) frame - source & destination MAC addresses fields
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This Ethernet frame was released by the three companies DEC, Inter, and Xerox. This frame version is the one that is commonly used today, for it has fields that align with the recent requirements of Ethernet technology. It has a structure that is, in most parts, the same as others. The following is the structure and the components of Ethernet II frame.
Destination MAC address :

To begin with Destination MAC address field (6 bytes), if you have read the Data-link Layer - Framing and addressing lesson, this field would seem much easier to you. However, we will adequately explain it again.

MAC address is an identifier that consists of twelve hexadecimal digits like 5F-8G-DD-5L-H6-6T. MAC address cannot be repeated globally, that is, each device has got a unique MAC address. This address is divided into two parts ; the first one, which is referred to as OUI (Organizational Unique Identifier), is for an organization and the second one for the NIC cards’ vendors. These rules are established by IEEE. Look at this example.


As shown in the above figure, any vendor that sells network devices is required to have an Organizational  Unique Identifier or OUI, such as in the above example 5F-8G-DD. Therefore, IEEE rules stipulate that OUI first 3 bytes must be included in all MAC address assigned to a given Ethernet device or NIC. In addition, All MAC addresses which have the same OUI must be assigned a unique value  in the last 3 bytes.

Sometimes MAC address is referred to as a burned-in address (BIA) because of the fact that it is burned  into read-only memory (ROM) on the NIC.  Being burned int the ROM means that it can never be changed.
The purpose of MAC address is to identify a network device within, for example, a local area network (LAN). It guides the  Ethernet frame towards the correct path until it reaches the destination.  Switches in a network are the devices that forward the arrived frames to their destined nodes.

Source MAC address :

Source MAC address, which is the second field occupying 6 bytes, is used to identify the sender of the frame. It is helpful in case the receiver intends to transmit back Ethernet frames. Once the Ethernet frame arrives at a switch device, this latter takes the MAC address and stores it in a list called CAM table or lookup table so that if the receiver wishes to transmit something  to the sending device, the switch would immediately forward the frame to this latter device.

The next lesson is a sequel to this one 
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10: Ethernet - Ethernet II (version 2, or DIX) frame - source & destination MAC addresses fields part 11 Reviewed by BOUFTIRA on 4:06:00 PM Rating: 5

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