The frame used by Ethernet technology is an Ethernet frame; the frame used by a token ring is a token ring frame; the frame used by FR technology is an FR frame, and so on and so forth. When we mention frames herein, unless specifically stated otherwise, it will always be in reference to Ethernet frames.
After completing this section, you should be able to: • Describe MAC address structures and classifications. • Describe the Ethernet II frame structure.
• Distinguish the differences and relationships between unicast, multicast, and broadcast MAC addresses and frames.
3.2.1
MAC Addresses
In February 1980, the IEEE convened a meeting to launch a major standardization project called IEEE Project 802. The 80 in 802 stands for 1980, while the 2 stands for February.
The project’s aim was to formulate a series of LAN standards. The Ethernet (IEEE 802.3), token bus (IEEE 802.4), token ring (IEEE 802.5), and other such LAN standards are the result of IEEE 802. As such, the standards developed under IEEE Project 802 are collectively referred to as IEEE 802 standards.
The IEEE 802 standard defines and standardizes the Medium Access Control (MAC) address for all network interface cards (such as Ethernet and token ring network cards) that must have a MAC address. Not every network interface card must have a MAC address. For example, SDH network interface cards do not have MAC addresses, and so do not comply with the IEEE 802 standard. Hereinafter, all references to network cards will refer to Ethernet cards.
Every network card has its own globally unique MAC address, which is 48 bits (6 bytes) in length and identifies the card. A network card manufacturer must register with the IEEE to obtain an Organizationally-Unique Identifier (OUI), which is 24 bits (3 bytes) in length and identifies the manufacturer. During card production, the manufacturer burns to the Read Only Memory (ROM) of each card the MAC address (referred to as a Burned-In Address, BIA). Thefirst three bytes of the BIA is the manufacturer’s OUI, with the last three bytes being determined by the manufacturer.
For different network cards produced by the same manufacturer, the last three bytes of the BIA must be different. The BIA of a network card cannot be altered and can only be read. Figure3.3shows the format of a BIA.
A BIA is a kind of unicast MAC address. MAC addresses can be divided into three types: unicast, multicast, and broadcast MAC addresses.
1. A unicast MAC address is identified by its least significant bit of the first byte being 0.
2. A multicast MAC address is identified by its least significant bit of the first byte being 1.
3. A broadcast MAC address is identified by all its bits being 1.
4. A unicast MAC address (such as a BIA address) identifies a network card; a multicast MAC address identifies a group of network cards; a broadcast MAC address is a special kind of multicast MAC address which identifies all network cards.
In Fig.3.4, we can see that only the unicast MAC address contains an OUI, whereas the multicast and broadcast MAC addresses do not, because only unicast MAC addresses identify individual network cards and multicast and broadcast MAC addresses are logical MAC addresses that identify multiple network cards.
A MAC address is often expressed in one of two common formats. In one of the formats, the MAC address is expressed as six groups of hexadecimal digits, with each group (1 byte) connected by a dash. In the other format, the MAC address is expressed as three groups of hexadecimal digits, with each group (2 bytes) connected by a dash. Figure3.5shows the two formats.
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6
OUI Vendor Assigned
Fig. 3.3 BIA format
XXXXXXX0 XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX OUI XXXXXXX1 XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX Non-OUI 11111111 11111111 11111111 11111111 11111111 11111111 Non-OUI Unicast MAC address Multicast MAC address Broadcast MAC address
Fig. 3.4 MAC address types and formats
3.2.2
Ethernet Frame Formats
There are two standard types of Ethernet frame formats: the IEEE 802.3 format, which is defined by IEEE 802.3, and the Ethernet II format (also called the DIX format), which is jointly defined by Digital Equipment Corporation (DEC), Intel, and Xerox. Figure3.6shows the two types of format.
Despite the differences between the two formats, current network devices sup- port both. The Ethernet II format is most widely used, whereas the IEEE 802.3 format is generally used only in certain scenarios where the frames are required to carry specific protocol information.
The following describes the differentfields in an Ethernet II frame:
1. Destination MAC address (6 bytes): contains the MAC address of the entity to which the frame is sent. A destination MAC address can be a unicast, multicast, or broadcast MAC address (frames containing such are called unicast, multicast, or broadcast frames, respectively).
2. Source MAC address (6 bytes): contains the MAC address of the entity from which the frame is sent. A source MAC address can only be a unicast MAC address.
3. Type (2 bytes): contains the payload data type. For example, a value of 0x0800 indicates that the payload data is an IPv4 packet, a value of 0x86dd indicates an IPv6 packet, a value of 0x0806 indicates an ARP packet, and a value of 0x8848 indicates an MPLS packet.
4. Payload Data (46–1500 bytes, variable): contains the payload of the frame. 5. CRC Field (4 bytes): contains a checksum (an integrity check) of the frame.
CRC stands for Cyclic Redundancy Check. The working mechanism of CRC is beyond the scope of this book.
The destination MAC address, source MAC address, type, payload data, and CRC fields are the same in both an Ethernet II and an IEEE 802.3 frame. The remainingfields in an IEEE 802.3 frame are beyond the scope of this book.
00000000 00011110 00010000 11011101 11011101 00000010 00000001 10000000 11000010 00000000 00000000 00000001 ff-ff-ff-ff-ff-ff or ffff-ffff-ffff 11111111 11111111 11111111 11111111 11111111 11111111 Unicast MAC address Multicast MAC address Broadcast MAC address 01-80-c2-00-00-01 or 0180-c200-0001 00-1e-10-dd-dd-02 or 001e-10dd-dd02
Fig. 3.5 MAC address formats
3.2.3
Review Questions
1. How many network cards can a network card manufacturer produce with a single OUI? (Choose one)
A. 1
B. 16,777,216 C. 256 D. 65,536
2. Which of the following identifies a MAC address of 05-1e-10-0d-d0-03? (Choose one)
A. Unicast MAC address B. Multicast MAC address C. Broadcast MAC address
3. Which of the following descriptions are correct? (Choose all that apply) A. The two types of Ethernet frame format are the IEEE 802.3 format and the
IEEE 802.4 format. Frame header Frame payload Frame trailer Destination MAC address Source MAC address Length DS AP SS APCtrl org code 0x00 Type Payload (38-1492 bytes) CRC 6 6 2 1 1 1 3 2 4 Type CRC IEEE 802.3 format Ethernet II format Frame header Frame payload Frame trailer 4 6 6 2 Destination MAC address Source MAC address Payload (46-1500 bytes)
Fig. 3.6 Two standard Ethernet frame formats
B. The two types of Ethernet frame format are the IEEE 802.3 format and the Ethernet II format.
C. The two types of Ethernet frame format are the Ethernet I format and the Ethernet II format.
D. The two types of Ethernet frame format are the IEEE 802.3 format and the DIX format.
4. Which of the following descriptions are correct? (Choose all that apply) A. The destination MAC address of an Ethernet frame can only be a unicast
MAC address.
B. The source MAC address of an Ethernet frame can only be a unicast MAC address.
C. The source MAC address of a multicast frame can only be a unicast MAC address.