Two Purposes of Physical Addresses in Ethernet Protocol

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Ethernet is a networking technology that is widely used for local area networks (LANs). It uses physical addresses, also known as MAC addresses, to identify devices at the data link layer or Layer 2 of the OSI model. The Ethernet protocol uses physical addresses for the following purposes:

A. To uniquely identify devices at Layer 2: Each device on an Ethernet network has a unique MAC address that is assigned by the device manufacturer. These addresses are globally unique, which means that no two devices on the planet should have the same MAC address. This allows devices to communicate with each other on the same network without any conflicts.

B. To allow detection of a remote device when its physical address is unknown: When a device wants to send a packet to another device on the same network, it needs to know the destination MAC address. If the destination MAC address is not already in the sender's ARP cache, it needs to perform an ARP request to find the MAC address of the destination device. This is done by broadcasting an ARP request packet to all devices on the network, asking if anyone knows the MAC address of the destination device. The destination device will respond with an ARP reply packet that contains its MAC address. Once the sender receives the ARP reply, it can add the MAC address to its ARP cache and use it to send packets to the destination device.

C. Ethernet protocol does not use physical addresses to differentiate a Layer 2 frame from a Layer 3 packet. Instead, it uses an Ethertype field in the Ethernet header to indicate the type of protocol used in the payload of the frame. For example, an Ethertype of 0x0800 indicates that the payload is an IPv4 packet.

D. Ethernet does not use physical addresses to establish a priority system to determine which device gets to transmit first. Instead, it uses a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) algorithm to manage access to the network medium. Devices listen to the network to determine if it is idle before transmitting, and if two devices transmit at the same time, they detect the collision and back off for a random amount of time before attempting to transmit again.

E. Ethernet uses physical addresses to allow communication between different devices on the same network. By using MAC addresses, Ethernet ensures that each device on the network has a unique identifier that can be used to send and receive packets.

F. Ethernet does not use physical addresses to allow communication with devices on a different network. To communicate with devices on a different network, Ethernet relies on higher-layer protocols such as IP (Internet Protocol) and routing protocols to route packets between networks.