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Difficulty: EasyIPv6 Addressing and Prefixes

An administrator enables IPv6 on a router interface using the prefix 2001:db8:feed:1::/642001:\text{db8}:\text{feed}:1::/64 with EUI-64 automatic interface identifier generation. If the interface MAC address is `1C1D.86AB.CDEF`, which IPv6 address is dynamically assigned to the interface?

  1. 2001:db8:feed:1:1e1d:86ff:feab:cdefAnswer
  2. B
    2001:db8:feed:1:1c1d:86ff:feab:cdef
  3. C
    2001:db8:feed:1:1e1d:86fe:ffab:cdef
  4. D
    2001:db8:feed:1:1c1d:86fe:ffab:cdef

Answer

2001:db8:feed:1:1e1d:86ff:feab:cdef
Generating an IPv6 EUI-64 interface identifier from a 48-bit MAC address requires splitting the MAC address into two 24-bit halves, inserting the hexadecimal sequence FFFE in the center, and flipping the 7th bit (Universal/Local bit) of the first byte. For MAC address 1C1D.86AB.CDEF, the first byte 0x1C (binary 00011100) becomes 0x1E (binary 00011110) after flipping the 7th bit. Inserting FFFE into the center yields the 64-bit interface ID 1e1d:86ff:feab:cdef, which combines with prefix 2001:db8:feed:1::/64 to form 2001:db8:feed:1:1e1d:86ff:feab:cdef.

Step-by-Step Solution

1
Split MAC address into two 24-bit halves
First half: 1C-1D-86, Second half: AB-CD-EF
EUI-64 insertion requires placing a 16-bit payload into the middle of the 48-bit MAC address.
2
Insert the 16-bit value FFFE into the center
1C-1D-86-FF-FE-AB-CD-EF
Standard EUI-64 processing expands 48-bit MAC addresses to 64-bit interface identifiers.
3
Invert the 7th bit (Universal/Local bit) of the first octet
0x1C (0001 1100) becomes 0x1E (0001 1110), yielding 1E-1D-86-FF-FE-AB-CD-EF
RFC 4291 requires flipping the 7th bit of the IEEE MAC address when converting to EUI-64 format.
4
Combine with IPv6 prefix
2001:db8:feed:1:1e1d:86ff:feab:cdef
Appending the 64-bit interface identifier to the /64 prefix constructs the complete IPv6 address.

Key Concept

EUI-64 Interface Identifier Generation
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Topics

IPv6 Addressing and Prefixes

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