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Difficulty: HardIPv6 Address Types and EUI-64

An engineer inspects the IPv6 neighbor discovery table on a Cisco router and observes the dynamically generated link-local address fe80::2e0:a3ff:fe14:5678 assigned to an auto-configured host interface. Based on the standard EUI-64 calculation process, what is the original burned-in MAC address of the host network interface card?

  1. 00e0.a314.5678Answer
  2. B
    02e0.a314.5678
  3. C
    00e0.a3ff.5678
  4. D
    04e0.a314.5678

Answer

The original burned-in MAC address of the host network interface card is 00e0.a314.5678.
To reverse the EUI-64 process for the interface ID 02e0:a3ff:fe14:5678, first remove the 16-bit 'FFFE' inserted into the middle, which leaves 02e0.a314.5678. Next, invert the 7th bit (Universal/Local bit) in the first byte. Hexadecimal 02 is binary 00000010; inverting the 7th bit produces binary 00000000 (hexadecimal 00). Therefore, the original MAC address is 00e0.a314.5678.

Step-by-Step Solution

1
Isolate the 64-bit interface identifier from the link-local IPv6 address
The interface identifier portion of fe80::2e0:a3ff:fe14:5678 is 02e0:a3ff:fe14:5678.
Link-local addresses append a 64-bit interface identifier to the fe80::/64 prefix.
2
Remove the 16-bit FFFE value inserted into the middle of the EUI-64 interface identifier
Removing 'ff:fe' from 02e0:a3ff:fe14:5678 leaves 02e0:a314:5678.
The EUI-64 process creates a 64-bit interface ID from a 48-bit MAC by inserting FFFE (1111 1111 1111 1110) between the 3rd and 4th bytes of the MAC address.
3
Invert the 7th bit (Universal/Local bit) of the first octet to restore the original MAC address
First octet 02 (0000 0010 binary) inverted at the 7th bit yields 00 (0000 0000 binary), resulting in MAC 00e0.a314.5678.
EUI-64 generation flips the 7th bit of the first byte (changing 0 for universal to 1 for local); reversing the process requires flipping this bit back.

Key Concept

Reversing standard IPv6 EUI-64 interface identifier generation to recover the original 48-bit MAC address
Estimated Time:1m 30s
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Topics

IPv6 Address Types and EUI-64

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