| How do stations identify each other?
In Ethernet, an application can choose to address the entire group,
a set of hosts, or a specific host within the scope of communication
(the Ethernet segment). Speaking to the group requires a broadcast;
contacting a set of individual stations requires a multicast; and addressing
one end system requires a unicast. Most traffic in a network is
unicast in nature, characterized as traffic from a specific station
to another specific device. Some applications generate multicast
traffic. Examples include multimedia services over LANs. These
applications intend for more than one station to receive the
traffic, but not necessarily all the stations.
Video conferencing applications
frequently implement multicast addressing to specify a group of
recipients. Networking protocols typically have a need to create
broadcast traffic in certain instances. For example, IP creates
broadcast packets for Address Resolution Protocol (ARP) requests.
Routers often transmit routing updates as broadcasts. AppleTalk,
Novell Internetwork Packet Exchange (IPX), and other Layer 3
protocols create broadcasts to perform name resolution and to carry
out various other functions.
The Figure shows a simple Ethernet
system with several devices attached. The Ethernet adapter card of
each device has a 48-bit (6-octet) address built into it that
uniquely identifies the station. This is called the Media Access
Control (MAC) address, or the hardware address. All the devices in a
LAN must have a unique MAC address. Devices express MAC addresses as
hexadecimal values. Sometimes MAC address octets are separated by
hyphens "-", sometimes by colons ":", and sometimes by periods
".".
The three formats, 00-60-97-8F-4F-86, 00:60:97:8F:4F:86, and
0060.978F.4F86, all specify the same host.
To help ensure uniqueness, the first
three octets indicate the vendor that manufactured the interface
card. This is known as the Organizational Unique Identifier (OUI).
Each manufacturer has a unique IEEE-assigned OUI value.
The last three octets of the MAC
address amount to a host identifier for the device. The last three
octets are assigned by the vendor. The combination of OUI and
"host number" creates a unique address for that device.
Each vendor is responsible to ensure that each of the Ethernet
adapters that it manufactures has a unique combination of six
octets.
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