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Questions and Answers
What does the vendor code in a MAC address represent?
What does the vendor code in a MAC address represent?
- A unique identifier assigned during manufacturing only
- A random number generated for each device
- A unique identifier assigned by the vendor (correct)
- A unique identifier assigned by the operating system
What can lead to the existence of duplicate MAC addresses?
What can lead to the existence of duplicate MAC addresses?
- Software updates
- Changes made to network protocols
- Mistakes in manufacturing or virtual machine implementations (correct)
- Proper manufacturing processes
What happens to the MAC address when a computer boots up?
What happens to the MAC address when a computer boots up?
- It is modified by the operating system
- It is erased from ROM
- It is copied from ROM to RAM (correct)
- It is generated randomly
Which term is often used to refer to the MAC address due to its storage method?
Which term is often used to refer to the MAC address due to its storage method?
What is a unicast MAC address used for in Ethernet communications?
What is a unicast MAC address used for in Ethernet communications?
Which of the following is NOT a reason to change a MAC address?
Which of the following is NOT a reason to change a MAC address?
What does the destination MAC address in an Ethernet frame identify?
What does the destination MAC address in an Ethernet frame identify?
What is the format of an Ethernet MAC address?
What is the format of an Ethernet MAC address?
Why is hexadecimal used to represent Ethernet MAC addresses?
Why is hexadecimal used to represent Ethernet MAC addresses?
What must vendors do to ensure their MAC addresses are unique?
What must vendors do to ensure their MAC addresses are unique?
Which layer of the OSI model does MAC addressing operate?
Which layer of the OSI model does MAC addressing operate?
What is the length of an Ethernet MAC address in bytes?
What is the length of an Ethernet MAC address in bytes?
What does OUI stand for in the context of MAC addressing?
What does OUI stand for in the context of MAC addressing?
What happens if two devices on a network have the same MAC address?
What happens if two devices on a network have the same MAC address?
How are hexadecimal numbers often denoted in documentation?
How are hexadecimal numbers often denoted in documentation?
Which of the following is true about changing MAC addresses?
Which of the following is true about changing MAC addresses?
What is the structure of an Ethernet MAC address?
What is the structure of an Ethernet MAC address?
What happens if two devices on a network have the same MAC address?
What happens if two devices on a network have the same MAC address?
At which layer of the OSI model does MAC addressing operate?
At which layer of the OSI model does MAC addressing operate?
Is it possible to change a device's MAC address?
Is it possible to change a device's MAC address?
What does the vendor OUI in a MAC address represent?
What does the vendor OUI in a MAC address represent?
What is the purpose of the FCS field in an Ethernet frame?
What is the purpose of the FCS field in an Ethernet frame?
How many bytes are contained in an Ethernet MAC address?
How many bytes are contained in an Ethernet MAC address?
What feature do most modern switches support regarding cable connections?
What feature do most modern switches support regarding cable connections?
What is encapsulated in an Ethernet frame?
What is encapsulated in an Ethernet frame?
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Study Notes
Ethernet MAC Address
- Each Ethernet device has a unique MAC address derived from a vendor-specific organizationally unique identifier (OUI).
- OUI is a 24-bit (3-byte) code assigned to vendors by IEEE, ensuring distinct MAC addresses across devices.
- An Ethernet MAC address is a 48-bit value, typically represented in hexadecimal as 12 digits (e.g., 00-60-2F-3A-07-BC).
- MAC addresses are often hardcoded into a device's read-only memory (ROM) and can be referred to as burned-in addresses (BIA).
- Changing a MAC address is possible via software on modern systems, which complicates the security of MAC-based filtering.
Ethernet Frame Processing
- The Ethernet frame comprises essential fields: preamble, destination MAC address, source MAC address, EtherType, data, and Frame Check Sequence (FCS).
- Frames are processed by examining the destination MAC address against the device's stored MAC in RAM; a match allows further processing up the OSI layers.
- Ethernet NICs accept frames with matching MAC addresses or if intended for a broadcast/multicast group.
Addressing and Communication Types
- Different types of MAC addressing in Ethernet: unicast (one-to-one), broadcast (one-to-all), and multicast (one-to-many).
- Each network device, including workstations, printers, and routers, has an Ethernet MAC address for identification.
Data Encapsulation and Cables
- Ethernet operates at the data link and physical layers, requiring various cable types for connections between similar and different devices.
- A crossover cable connects like devices, while straight-through cables connect unlike devices; auto-MDIX feature can auto-detect cable types.
Frame Forwarding Methods on Switches
- Store-and-Forward Switching: Entire frame is received; switch computes the CRC to check for errors before forwarding. Error detection helps prevent bandwidth waste with corrupt data.
- Cut-Through Switching: Forwarding begins upon reading the destination address, potentially leading to faster performance but does not check for frame integrity before transmission.
Network Configuration and Operations
- Upon boot, a NIC loads its MAC address from ROM into RAM, facilitating network communication.
- In environments with a default gateway, Ethernet frames are sent to the gateway MAC address when IP is on a different network.
- Switches dynamically learn MAC addresses as they receive frames, populating their MAC address table—also known as a CAM table—for efficient frame forwarding.
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