Computing the IPv6 EUI-64 Interface ID from a Given MAC
Short answer
D EUI-64 inserts FFFE into the MAC's middle and inverts the seventh bit, producing 2001:db8:3bb8:3bb1:C801:B3FF:FEB8:1.
Question
Key Takeaways
The FFFE marker and the inverted seventh bit are what make a correct EUI-64 address recognizable among the answer options.
EUI-64 builds the 64-bit interface ID from the 48-bit MAC: split the MAC at the OUI/host boundary, insert FFFE in the middle, and flip the seventh bit (U/L bit) of the first byte. The result appends to the configured /64 prefix.
Common mistake: Forgetting to invert the seventh bit or misplacing FFFE. A correct EUI-64 always shows FFFE in the fourth hextet; options lacking it are wrong.
Expert Analysis
Why the Answer Is Correct
D is correct. EUI-64 takes the 48-bit MAC, splits it, inserts FFFE between the OUI and host halves, and inverts the universal/local bit (the seventh bit of the first byte). The resulting interface ID 2001:db8:3bb8:3bb1:C801:B3FF:FEB8:1 is the only option containing FFFE in the expected position.
Why the Other Options Are Wrong
A, B, and C are wrong because they lack the FFFE insertion that EUI-64 always places in the fourth hextet, or they fail to reflect the inverted seventh bit. EUI-64 addressing is deterministic, so any option without FFFE cannot be produced by the router.
Community Comment Notes
Commenters note D is "the only answer with FFFE in it," and one explains the seventh-bit flip using a hex table. The community consensus (89 votes for D) aligns with the EUI-64 rule.
Official Reference
https://learningnetwork.cisco.com/s/ccna-exam-topics
This link opens the official vendor documentation in a new tab.
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 5 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 5
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picho707 👍 4 Answered DThis is the only answer with FFFE in it.
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Isuckatexams 👍 30 | 1 2 | 3 --- 4|5 6|7 --- 8|9 A|B --- C|D E|F use this table in your head to flip the 7th bit. 0 -> 2 or 2 -> 0 etc.
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MinSun600 👍 1why the a transfered to 8 ?
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Rolgui 👍 1 Answered DEUI-64 format
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Iidear 👍 1 Answered DD: Correct
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