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.
Figure 1 : Ethernet/MAC Addressing
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 Ã
Size in Bytes ß 3 bytes à | ß 3 bytes Ã
Size in Hex ß 6 Hex Digit à | ß 6 Hex Digits Ã
Example ß 00 30 2F à | ß 4A 07 BC Ã
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.
