Identifying Invalid Unicast MAC Addresses Based on I/G Bit

H12-811 By islever Editorial Last updated

Data Communication and Network Basics

Short answer

BD Both contain a Group bit of 1 in the first octet, designating them as multicast addresses unsuitable for host network adapters.

Question

Which of the following MAC addresses cannot be used as the MAC addresses of host network adapters?
  1. 00-02-03-04-05-06
  2. 01-02-03-04-05-06 Correct Answer
  3. 02-03-04-05-06-07
  4. 03-04-05-06-07-08 Correct Answer

Key Takeaways

The question tests knowledge of the MAC address structure, specifically the Individual/Group (I/G) bit in the first octet which determines if an address is unicast (0) or multicast (1).

Learn how to identify invalid MAC addresses for host network adapters by checking the Individual/Group bit. Correct answers are B and D because they are multicast addresses.

Common mistake: Candidates often confuse the Universal/Local (U/L) bit with the I/G bit. They may incorrectly reject addresses like 02-xx-xx-xx thinking they are invalid, while missing that 01-xx-xx-xx and 03-xx-xx-xx are explicitly multicast.

Expert Analysis

To determine which MAC addresses cannot be assigned to a host network adapter, one must examine the Individual/Group (I/G) bit located in the least significant bit (LSB) of the first octet. This bit distinguishes between unicast and multicast/broadcast addresses. If the I/G bit is set to 1, the address is a group address (multicast or broadcast), which is intended for multiple recipients rather than a single specific interface. Host network adapters are logically assigned unicast addresses where the I/G bit is 0. Conversely, if the I/G bit is 0, it indicates a globally unique or locally administered unicast address suitable for a physical NIC.

Analyzing the options based on their first octets:

  • Option A (00-02...): The binary value 0000 0000 has an LSB of 0. This is a valid unicast address.
  • Option B (01-02...): The binary value 0000 0001 has an LSB of 1. This identifies it as a multicast address. Therefore, it cannot be used as a host's permanent hardware identifier.
  • Option C (02-03...): The binary value 0000 0010 has an LSB of 0. This is a valid unicast address.
  • Option D (03-04...): The binary value 0000 0011 has an LSB of 1. Like Option B, this is a multicast address and cannot serve as a host's primary MAC address.

Community comments correctly highlight that both B and D represent multicast addresses due to the set LSB. While some discussions might focus solely on 'all-zeros' or 'all-ones' special cases, the fundamental rule prohibiting any multicast address from being a host's unique adapter ID applies here. Thus, both B and D are invalid for this purpose.

Exam Strategy

When analyzing MAC address validity questions, always convert the first octet to binary to inspect the two critical bits: the Least Significant Bit (I/G) and the Second Least Significant Bit (U/L). Remember that a host NIC requires a unicast address (I/G = 0). Any option with an odd-numbered first octet (ending in 1, 3, 5, etc.) immediately flags a multicast address, making it invalid for a unique host assignment.

Frequently Asked Questions

What does the first bit of a MAC address indicate?

The least significant bit of the first octet is the Individual/Group (I/G) bit. A value of 0 indicates a unicast address, while 1 indicates a multicast or broadcast address.

Why can't a host use a multicast MAC address?

Multicast addresses are designed to reach a group of devices simultaneously. A host network adapter needs a unique unicast address to receive frames specifically addressed to it alone.

More H12-811 FAQ →

Community Votes

B
100%

100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 6 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 6

  1. Allenrok 👍 2
    Option B (01-02-03-04-05-06) starts with 01, which in binary is 0000 0001. The least significant bit of the first octet in a MAC address, when set to 1, indicates a multicast address. Host network adapters typically use unicast addresses, so this MAC address would not typically be used for a host network adapter.
  2. andrpercud 👍 2 Answered B
    B and D are correct,
  3. Silveira 👍 2 Answered B
    CORRETA: (B)
  4. Fmoura17 👍 1 Answered B
    Why B and D Cannot Be Used The least significant bit (I/G bit) of the first octet determines whether a frame is destined for a single host (unicast) or a group (multicast/broadcast): 01 (B) — I/G bit = 1 → This is a multicast address. Network adapters cannot be assigned a multicast MAC as their own address. 03 (D) — I/G bit = 1 → Same reason, also a multicast address. A host NIC must have a unicast MAC address (I/G bit = 0), so any MAC address with an odd first octet is reserved for multicast/broadcast and cannot be assigned to a host adapter.
  5. HCIA 👍 1
    So, option D (03-04-05-06-07-08) is also correct, as its least significant bit of the first octect is set to 1.
Show 1 more comment
  1. hafizhou 👍 1
    C correct

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