Broadcasts and Multicasts
With Ethernet, broadcasts are specialized frames that are
destined for all devices on an Ethernet network. Broadcasts use a MAC address of
FF-FF-FF-FF-FF-FF. This is a special MAC address because it is the highest
number allowed in the 48-bit schema of MAC addresses. In binary, all 48 bits are
set to 1.
Multicasts are specialized broadcasts. Multicasts are used by
higher layer protocols to direct traffic to more than one select destination,
rather than a broadcast, which is sent to all destinations. Application layer
multicasts start with 01-00-5E prefix. The rest of the digits are assigned by
the application layer protocol handling the multicast. However, other Layer
2-only multicast addresses do not have the prefix of 01-00-5E; for example, STP
with MAC address of 01-00-0c-cc-cc-cd. For the most part, Ethernet networks
treat multicasts like broadcasts by default. Several higher layer protocols,
such as IGMP (Internet Group Messaging Protocol), can be used by switches to
differentiate the traffic and forward only multicast out specific ports.
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