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Difficulty: MediumIPv6 Addressing and Prefixes

A network administrator executes the command `show interface GigabitEthernet0/0` on a Cisco router and notes a MAC address of `a1b2.c3d4.e5f6`. The interface is then configured with the command `ipv6 address 2001:db8:1111:2222::/64 eui-64`. Which IPv6 global unicast address will be generated on this interface?

  1. 2001:db8:1111:2222:a3b2:c3ff:fed4:e5f6Answer
  2. B
    2001:db8:1111:2222:a1b2:c3ff:fed4:e5f6
  3. C
    2001:db8:1111:2222:a3b2:c3fe:ffd4:e5f6
  4. D
    2001:db8:1111:2222:a3b2:c3d4:e5f6:fffe

Answer

2001:db8:1111:2222:a3b2:c3ff:fed4:e5f6
Generating an IPv6 EUI-64 interface identifier requires splitting the 48-bit MAC address (a1:b2:c3:d4:e5:f6) into two 24-bit halves, inserting FFFE in the middle (a1b2:c3ff:fed4:e5f6), and flipping the 7th bit (Universal/Local bit) of the first byte. Converting 0xa1 (10100001 in binary) to invert the 7th bit yields 0xa3 (10100011 in binary). Combining the prefix 2001:db8:1111:2222::/64 with the modified interface ID gives 2001:db8:1111:2222:a3b2:c3ff:fed4:e5f6.

Step-by-Step Solution

1
Split MAC address into OUI and NIC identifier halves
OUI portion is a1:b2:c3 and NIC portion is d4:e5:f6
EUI-64 insertion occurs directly between the 24-bit OUI and the 24-bit device identifier.
2
Insert the 16-bit hex value FFFE between the two halves
a1b2:c3ff:fed4:e5f6
EUI-64 expands a 48-bit MAC address into a 64-bit interface identifier by adding 0xFFFE.
3
Invert the 7th bit (Universal/Local bit) of the first byte (0xa1)
0xa1 in binary is 10100001. Inverting the 7th bit yields 10100011, which is 0xa3.
IEEE EUI-64 standard requires flipping the Universal/Local (U/L) bit so that 0 (universal) becomes 1 (locally administered scope for auto-configured interfaces).
4
Combine configured /64 prefix with generated 64-bit interface ID
2001:db8:1111:2222:a3b2:c3ff:fed4:e5f6
The prefix 2001:db8:1111:2222::/64 forms the first 64 bits and the modified EUI-64 ID forms the last 64 bits.

Key Concept

EUI-64 Interface Identifier Generation
Estimated Time:1m 30s
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Topics

IPv6 Addressing and Prefixes

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