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Ethernet Addressing


Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as hardware address or MAC addressing. It is a 48-bit number used to uniquely identify each computer in the internet. The address is usually written in hexadecimal form (e.g. 01:23:45:67:89:ab). The first three octets are known as the Organizationally Unique Identifier (OUI) -- an identifier issued by IEEE to uniquely identify the ethernet product manufacturer. The following three octets are assigned by that organization to guarantee uniqueness among their own products. As a result, the combination guarantees that each NIC in the world is uniquely identified. Ethernet address has many other names: called MAC address, hardware address, physical address, burned-in address (BIA), universal address and unicast Ethernet address.

Unicast Ethernet address emphasis the fact that the address represents one interface to the Ethernet LAN. There are broadcast and multicast address.

unicast, multicast and broadcast
unicast, multicast and broadcast

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.

The NIC provides a RJ-45 ports for ethernet RJ-45 connector.

The NIC itself 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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