150-130 · Question #111
The multicast IP address 239.10.20.30 is mapped to which MAC address?
The correct answer is D. 01-00-5e-0a-14-1e. See the full explanation below for the reasoning.
Question
The multicast IP address 239.10.20.30 is mapped to which MAC address?
Options
- A00-00-ef-0b-15-1f
- B00-00-5e-0a-14-1e
- C01-00-ef-0b-15-1lf
- D01-00-5e-0a-14-1e
How the community answered
(30 responses)- A7% (2)
- B10% (3)
- C3% (1)
- D80% (24)
Community Discussion
10
Prof. SaraJun 1, 2026The answer is D, 01-00-5e-0a-14-1e. The rule to memorize is this: all IPv4 multicast MAC addresses begin with 01-00-5e, that prefix is hardwired by the IEEE, and any option that starts with 00-00 is dead on arrival because the first octet must have its low-order bit set to 1, which is the multicast bit. From there you drop the high-order bit of the second IP octet and convert the remaining three significant octets straight to hex, so 10 becomes 0a, 20 becomes 14, and 30 becomes 1e, giving you 01-00-5e-0a-14-1e exactly. Option C uses the right prefix but has a typo in the last byte that makes it invalid hex, and option B swaps in 00-00-5e which violates the multicast bit rule. Think of the mnemonic "ONE-ZERO-FIVE-E starts the multicast tree" to lock in that opening sequence every time this question shows up on the 150-130 blueprint under Layer 2 addressing.
Toby R.Jun 1, 2026That mnemonic is solid but what locked it in for me was remembering that the low-order bit of the first octet being 1 is the same multicast bit rule at Layer 2 that makes FF:FF:FF:FF:FF:FF a broadcast, so you are really just learning one underlying concept, not two separate facts.
Wesley A.Apr 23, 2026D is correct, 01-00-5e-0a-14-1e. The first trap here is the prefix, every IPv4 multicast MAC starts with 01-00-5e, which rules out A and B immediately, and C uses ef instead of 5e so toss that one too. Then you map the low 23 bits of the IP address into the last three octets of the MAC, so 10 becomes 0a, 20 becomes 14, and 30 becomes 1e, landing you right on D.
Viktor S.Apr 24, 2026The 23-bit mapping is the part worth drilling, because the upper bit of the last octet of the group address gets dropped, which means two different multicast IPs can share the same MAC and cause hosts to receive traffic they never asked for.
Brenda K.May 21, 2026Quick win, bank the seconds. The rule is mechanical: multicast MACs always start 01-00-5e, then you map the low 23 bits of the IP. Drop the first octet, zero out the high bit of the second octet, and you get 0a-14-1e, giving you 01-00-5e-0a-14-1e. The only distractor worth a second glance is B, because it gets the last three octets exactly right but starts with 00-00-5e instead of 01-00-5e, and that leading 01 is the whole point, it is what flags the frame as multicast at Layer 2. If you blanked on the OUI, eliminate A and C immediately since neither starts with 01-00-5e, then pick between B and D on the prefix alone. Answer is D, and you should not have spent more than 20 seconds here.
Yusuf A.May 24, 2026Good breakdown, and the thing my senior pointed out when he showed me this is that because only 23 of the 28 usable Class D bits get mapped into that MAC, up to 32 different multicast group addresses can land on the same Layer 2 address, which some harder questions use as a follow-up trap once you already know the 01-00-5e prefix cold.
Toby R.Jun 5, 2026Saw this exact one, just strip the first octet and swap 01-00-5e in front.
Yusuf A.Jun 16, 2026Got this almost exact question on my actual 150-130 attempt last month and I second-guessed myself for a solid two minutes before I remembered what a senior on my team drilled into me. Every IPv4 multicast MAC starts with 01-00-5e, that is the IEEE block reserved for multicast, so you can already knock out A and B right away since they both start with 00-00 instead. After that prefix, you take the low 23 bits of the IP address, which for 239.10.20.30 gives you 0a-14-1e in hex (10, 20, and 30 just convert straight across with a careful eye on that leading zero). That locks in D, 01-00-5e-0a-14-1e, and C is just a distractor with a typo in the last octet anyway, that "1lf" should have tipped me off immediately. I ended up circling back to flag it for review, but the prefix rule is what saved me, once you see 01-00-5e you know you are in the right ballpark.
Ola B.Apr 5, 2026Saw almost this exact address on my exam and the only thing that saved me was the lab I spun up the night before watching Wireshark capture multicast frames and confirming that prefix always starts with 01-00-5e, then you drop the first bit of the second octet and map the remaining 23 bits straight across, which locks you into D.
Viktor S.Apr 18, 2026D, obviously. A and C use the wrong OUI prefix, B misses the multicast bit.