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Difficulty: MediumIPv6 Address Types and EUI-64

An network engineer is configuring a Cisco router interface using IPv6 link-local autoconfiguration. The burned-in MAC address of the interface is `0800.279f.a1b2`. Which IPv6 link-local address will be automatically generated for this interface using the EUI-64 standard?

  1. fe80::a00:27ff:fe9f:a1b2Answer
  2. B
    fe80::800:27ff:fe9f:a1b2
  3. C
    fe80::a00:27ff:ffff:a1b2
  4. D
    fe80::a00:279f:a1b2:fffe

Answer

fe80::a00:27ff:fe9f:a1b2
The address fe80::a00:27ff:fe9f:a1b2 is correct because the EUI-64 process takes the MAC address 0800.279f.a1b2, inserts FFFE between the 24th and 25th bits (0800:27FF:FE9F:A1B2), and flips the 7th bit of the first byte from 0 to 1 (converting 08 to 0A). Combining this 64-bit interface ID with the FE80::/64 link-local prefix yields fe80::a00:27ff:fe9f:a1b2.

Step-by-Step Solution

1
Split the 48-bit MAC address into two 24-bit halves.
Left half: 0800.27, Right half: 9f.a1b2
EUI-64 requires inserting a 16-bit payload into the center of the 48-bit hardware address.
2
Insert the hex string FFFE between the two halves.
0800:27FF:FE9F:A1B2
FFFE is the standard 16-bit sequence specified by IEEE EUI-64 calculation.
3
Invert the 7th bit (Universal/Local bit) of the first byte.
First byte 08 (hex 0000 0000 -> 0000 1000) becomes 0A (hex 0000 1010). Resulting Interface ID: 0A00:27FF:FE9F:A1B2 (shortened to a00:27ff:fe9f:a1b2).
Inverting the U/L bit indicates that the address is globally unique / universally administered.
4
Prepend the IPv6 link-local prefix FE80::/64 to the interface ID.
fe80::a00:27ff:fe9f:a1b2
IPv6 link-local addresses use the prefix FE80::/64 combined with the 64-bit EUI-64 interface identifier.

Key Concept

EUI-64 Interface Identifier Generation
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Topics

IPv6 Address Types and EUI-64

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