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Ethernet Addressing

Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly refered to as hardware address or MAC addressing. Ethernet address, also called MAC address, hardware address, physical address, burned-in address (BIA) is a 48-bit number used to uniquely identify each computer in a network. The address is usually written in hexadecimal form (e.g. 01:23:45:67:89:ab). The first three octets identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI). The following three octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness.

If the least significant bit of the most significant byte is set to a 0, the packet is meant to reach only one receiving NIC. This is called unicast. If the least significant bit of the most significant byte is set to a 1, the packet is meant to be sent only once but still reach several NICs. This is called multicast. Packets sent to the broadcast address, all one bits, are received by all stations on a local area network. In hexadecimal the broadcast address would be "FF:FF:FF:FF:FF:FF". 

Do not confuse an Ethernet address with an Internet Protocol ("IP") address.

A network interface controller (NIC) is a hardware device that handles an interface to a computer network and allows a network-capable device to access that network. The NIC has a ROM chip that contains a unique number, the media access control (MAC) Address. The MAC address identifies the device uniquely on the LAN. The NIC exists on both the Physical Layer (Layer 1) and the Data Link Layer (Layer 2) of the OSI model.

nic

The NIC plugs into a motherboard and communicates with the network via a serial port and with motherboard via a parallel connection.

There are three techniques used to transfer data, the NIC may use one or more of these techniques.

  • Interrupt request line (IRQ): IRQ is a signal sent from an auxiliary device to the microprocessor, informing that an event happened on auxiliary device and CPU attention is needed. NIC will send IRQs during data transmission. 

  • input/output (I/O) address: a memory space within the operation system used by an auxiliary device to exchange data with the computer.

  • Direct Memory Access (DMA): DMA  is where an intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.


There are a few considerations when choosing the NICs:

  • The bandwidth supported by the NIC.

  • The type of media supported by the NIC.

  • The type of network architecture supported by the NIC.

  • The drivers (software) required by the NIC.


ICND1 and ICND2 break down

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