Mapping IPv4 Multicast IP to Ethernet MAC Address
Short answer
C The corresponding multicast MAC address is 01-00-5E-00-01-01, derived from the lower 23 bits of the IPv4 multicast group 224.0.1.1 mapped to the IEEE 802 standard prefix.
Question
Key Takeaways
IPv4 multicast MACs use a fixed 24-bit prefix (01-00-5E) and map only the lower 23 bits of the IP address, discarding the top 5 bits of the IP.
Determine the Ethernet MAC address for IPv4 multicast group 224.0.1.1 using the standard 23-bit mapping algorithm. The correct MAC address is 01-00-5E-00-01-01.
Common mistake: Option B (01-00-5E-00-01-00) is wrong because it fails to include the last bit of the IP address (the host portion .1). Option A uses an incorrect intermediate byte due to misalignment of the 23-bit boundary.
Expert Analysis
The mapping of IPv4 multicast addresses to Ethernet MAC addresses follows a standardized algorithm defined in RFC 1112. To convert an IP multicast address into a MAC address, the first 24 bits of the MAC are fixed as 01-00-5E, followed by a zero bit in the most significant position of the 25th bit (making it a valid unicast prefix for the lower half), and finally the lower 23 bits of the IPv4 multicast address.
First, examine the destination IP address: 224.0.1.1. In binary, this is:
11100000.00000000.00000001.00000001
Next, extract the lower 23 bits of this IP address. The first 5 bits (11100) are ignored because they correspond to the 224.x.x.x range which maps to the fixed 01-00-5E prefix. The remaining bits are:
- Bit 6-8 of Octet 2:
000(from00000000) - All 8 bits of Octet 3:
00000001 - All 8 bits of Octet 4:
00000001
Combining these yields the lower 24 bits of the MAC address (ignoring the leading zero): 00000000.00000001.00000001. Converting this to hexadecimal gives 00-01-01.
Appending this to the fixed prefix 01-00-5E results in the final MAC address: 01-00-5E-00-01-01. This matches Option C. A common trap is to incorrectly map all octets or ignore the 23-bit truncation rule, leading to incorrect hex values like those in options A, B, or D.
Official Reference
https://www.rfc-editor.org/rfc/rfc1112
This link opens the official vendor documentation in a new tab.
Exam Strategy
When dealing with multicast MAC addresses, always remember the 23-bit truncation rule. Write down the IP in binary, cross out the first 5 bits, and take the remaining 23 bits. Convert these directly to hex for the last three bytes of the MAC. Do not attempt to map full octets; the boundary often splits an octet.
Frequently Asked Questions
Why are only 23 bits of the IP address used?
Does every IPv4 multicast IP have a unique MAC?
Community Votes
100% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 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 1
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HCIP Resolutor 👍 1 Answered CLos últimos 3 octetos de 224.0.1.1 son 0.1.1 (00000000.00000001.00000001). Tomamos los últimos 23 bits (el bit más a la izquierda del segundo octeto se ignora): Octeto 2 (parcial, 7 bits): 0000000 = Hex: 00 Octeto 3 (8 bits): 00000001 = Hex: 01 Octeto 4 (8 bits): 00000001 = Hex: 01 01-00-5E + 00-01-01 = 01-00-5E-00-01-01
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