VigorSwitch G1260 User’s Guide
22
The MAC is belonged to Data Link Layer (Layer 2), the address is defined to be a 48-bit
long and locally unique address. Since this type of address is applied only to the Ethernet
LAN media access control (MAC), they are referred to as MAC addresses.
The first three bytes are Organizational Unique Identifier (OUI) code assigned by IEEE.
The last three bytes are the serial number assigned by the vendor of the network device. All
these six bytes are stored in a non-volatile memory in the device. Their format is as the
following table and normally written in the form as aa-bb-cc-dd-ee-ff, a 12 hexadecimal
digits separated by hyphens, in which the aa-bb-cc is the OUI code and the dd-ee-ff is the
serial number assigned by manufacturer.
Bit 47
Bit 0
1
st
byte
2
nd
byte
3
rd
byte
4
th
byte
5
th
byte
6
th
byte
OUI code
Serial number
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 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.
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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
DA
SA
Type/Length
Data
Pad bit if any
FCS
7 7
6
6 2
46-1500
4
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.
Destination address (DA)
- The DA field is used to identify which network device(s)
should receive the packet. It is a unique address. Please see the section of MAC addressing.
