CCNA RnS #11: Ethernet/MAC Addressing

CCNA RnS Article – Ethernet/MAC Addressing

In this article let’s talk about Ethernet Address also known as LAN Address, Hardware Address, Burned-in address, physical address, universal address, or MAC address.

We cannot imagine that an ethernet network can work without Ethernet Address. A layer 2 frame has Source Address and Destination Address fields. Sending device puts its own Ethernet Address in the Source Address field and Destination Address is the address of the intended receiver.

An Ethernet address is a 6-byte long binary number and if we convert it is bits it is 48 bits. For network admin convenience, it is represented as a 12-digit hexadecimal number.

You can find the Ethernet Address of your PC by typing the “ipconfig /all” command in the command prompt. In the output, search for the Physical Address which is the Ethernet Address for your system.

Point to note, there can be multiple network cards on your laptop so you will have multiple Physical Addresses in the output – one for each network card.

Ethernet/MAC Addressing
Figure 1 : Ethernet/MAC Addressing

You may find that the different manufacturers use different formats for representing ethernet addresses. In my case (figure 1), for every 2 hexadecimal values there is a hyphen (-), and for Cisco, after every 4 hexadecimal values, there is a dot (.).

Every network interface (RJ-45 or Wireless) has one Ethernet address assigned. For an Ethernet network to deliver traffic, it must be unique across the LAN network. If a frame needs to be sent to an Ethernet Address that is configured on two devices there will be confusion about whom to send this traffic to.

This uniqueness is ensured by IEEE. Every OEM who manufactures network devices gets a unique number for its organization. This unique number is 3 bytes long. This 3-byte long number is called Organizational Unique Identifier (OUI) and all Ethernet Addresses assigned to each device start with this OUI number. The last 3 bytes are controlled by the organization and ensure this is not assigned to any other card before. The last 3 bytes are also called Local Unique Identifier (LUI) or Vendor Assigned number.

The structure of an Ethernet Address looks like the below –

                             OUI                      LUI

Size in Bits          ß24 Bits à | ß 24 Bits à

Size in Bytes       ß 3 bytes à | ß 3 bytes à

Size in Hex         ß 6 Hex Digit à | ß 6 Hex Digits à

Example              ß 00 30 2F à | ß 4A 07 BC à

Every NIC has one Ethernet address assigned. Therefore, it is called the Unicast Ethernet Address. However, Ethernet LAN also employs other types of addresses that represent a group of devices in the network. Multicast Address refers to the set of devices in a LAN network and Broadcast Address refers to all the hosts on the network. Broadcast Ethernet Address is ffff.ffff.ffff.

Hope you find this informative.

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