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Bit 47 bit 0
1st byte 2nd byte 3rd byte 4th byte 5th byte 6th byte
OUI code Serial number
Table 3-3 Ethernet MAC address
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 addresses 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’s
interface that 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 bits
of DA are 1s, it is a broadcast, which means all network devices 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. A basic Ethernet frame format contains seven fields as
explained below:
PRE SFD DA SA Type/Length Data Pad bit if any FCS
7 7 6 6 2 46-1500 4
Fig. 3-3 Ethernet frame structure
- 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
ones. 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 is the left-most bit of the left-most byte of the destination
address. The SFD pattern is 10101011.
-