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Bit 47
1st byte
2nd byte
OUI code
The first bit of the first byte in the Destination address (DA) determines the
address to be a Unicast (0) or Multicast frame (1), known as I/G bit indicating
individual (0) or group (1). So the 48-bit address space is divided into two portions,
Unicast and Multicast. The second bit is for global-unique (0) or locally-unique
address. The former is assigned by the device manufacturer, and the later is usually
assigned by the administrator. In practice, global-unique addresse s are always
applied.
A unicast address is identified with a single network interface. With this
nature of MAC address, a frame transmitted can exactly be received by the target
an interface the destination MAC points to.
A multicast address is identified with a group of network devices or network
interfaces. In Ethernet, a many-to-many connectivity in the LANs is provided. It
provides a mean to send a frame to many network devices at a time. When all bit of
DA is 1s, it is a broadcast, which means all network device except the sender itself
can receive the frame and response.
Ethernet Frame Format
There are two major forms of Ethernet frame, type encapsulation and length
encapsulation, both of which are categorized as four frame formats 802.3/802.2
SNAP, 802.3/802.2, Ethernet II and Netware 802.3 RAW. We will introduce the
basic Ethernet frame format defined by the IEEE 802.3 standard required for all
MAC implementations. It contains seven fields explained below.
PRE
SFD
7
7
Preamble (PRE) —The PRE is 7-byte long with alternating pattern of
-
ones and zeros used to tell the receiving node that a frame is coming,
and to synchronize the physical receiver with the incoming bit stream.
The preamble pattern is:
10101010 10101010 10101010 10101010 10101010 10101010 10101010
Start-of-frame delimiter (SFD) — The SFD is one-byte long with
alternating pattern of ones and zeros, ending with two consecutive
1-bits. It immediately follows the preamble and uses the last two
consecutive 1s bit to indicate that the next bit is the start of the data
packet and the left-most bit in the left-most byte of the destination
address. The SFD pattern is 10101011.
3rd byte
Table 3-3 Ethernet MAC address
DA
SA
Type/Length
6
6
Fig. 3-3 Ethernet frame structure
27
4th byte
5th byte
Serial number
Data
Pad bit if any
2
46-1500
Publication date: January, 2005
User Manual
bit 0
6th byte
FCS
4
Revision A1
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