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Difficulty: MediumIPv6 Address Types and EUI-64

A Cisco router interface GigabitEthernet0/0/0 has the burned-in MAC address 1866.da34.5678. The interface is configured with the command `ipv6 enable` to automatically derive its IPv6 link-local address using stateless address autoconfiguration (SLAAC) and the IEEE EUI-64 process. Which IPv6 address will be assigned to the interface?

  1. fe80::1a66:daff:fe34:5678Answer
  2. B
    fe80::1866:daff:fe34:5678
  3. C
    fe80::1a66:daff:ffff:5678
  4. D
    fc00::1a66:daff:fe34:5678

Answer

fe80::1a66:daff:fe34:5678
The address fe80::1a66:daff:fe34:5678 correctly follows the IEEE EUI-64 interface identifier derivation by inserting FFFE into the middle of MAC address 1866.da34.5678, inverting the 7th bit of the first byte (changing 18 to 1A in hexadecimal), and prefixing it with the link-local scope fe80::.

Step-by-Step Solution

1
Split the 48-bit MAC address into two 24-bit halves and insert FFFE in the middle.
The MAC address 1866.da34.5678 (18:66:DA:34:56:78) becomes 1866:DAFF:FE34:5678.
EUI-64 standard requires expanding a 48-bit MAC address into a 64-bit interface identifier by placing FFFE between the Organizationally Unique Identifier (OUI) and the NIC-assigned extension.
2
Invert the 7th bit (Universal/Local bit) of the first byte.
The first byte 0x18 (binary 00010000) becomes 0x1A (binary 00011010). The expanded interface identifier becomes 1A66:DAFF:FE34:5678.
IEEE EUI-64 rules dictate that the 7th bit of the interface identifier must be inverted (0 indicates local scope, 1 indicates globally unique/universal).
3
Combine the IPv6 link-local prefix with the calculated 64-bit interface identifier.
Prefix fe80:: + 1a66:daff:fe34:5678 = fe80::1a66:daff:fe34:5678.
The `ipv6 enable` command dynamically creates a link-local address starting with the fe80::/10 prefix.

Key Concept

IPv6 EUI-64 Interface Identifier Derivation
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Topics

IPv6 Address Types and EUI-64

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