Figure 16-5 Ethernet Frame Format
Preamble: This field consists of 8 bytes of alternating 1s and 0s that
are used to synchronize the signals of the sender and receiver.
Destination Address: This field contains the address of the receiver.
Source Address: This field contains the address of the source device.
Type: This field contains a code that identifies the network layer
protocol.
Data: This field contains the data that was received from the network
layer and that needs to be transmitted to the receiver.
Frame Checksum Sequence (FCS): This 4-byte field includes a
checksum mechanism to ensure that the frame has been transmitted
without corruption.MAC Addresses
Media Access Control (MAC) addresses are used to
enable communication between devices connected to a
local network. Several types of MAC addresses are used
to accommodate the different types of network
communications. There are three major types of network
communications:
Unicast: In this type of communication, data frames are sent between
one specific source and addressed to one specific destination. This type
of transmission has one sender and one receiver, and it is the most
common type of traffic on any network.
Broadcast: In this type of communication, data frames are sent from
one source address to all other addresses connected on the same LAN.
There is one sender, but the information is sent to all devices connected
to the network.
Multicast: In this type of communication, information is sent from
one device to a group of devices or clients. In order for the clients to
receive the multicast data, they have to be members of a multicast
group. Whereas broadcast is used to transmit data to all the clients on
the network, multicast is used to transmit data to just a subset of those
clients.In order to distinguish between these three types of
network communications, the IEEE has defined MAC
addresses for each type: