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Difficulty: MediumIPv6 Address Types and EUI-64

A network engineer configures SLAAC on a dual-stack Cisco IOS router interface with a burned-in MAC address of `a036.9f12.3456`. Based on standard IEEE EUI-64 calculations, which IPv6 link-local address is automatically assigned to this interface?

  1. fe80::a236:9fff:fe12:3456Answer
  2. B
    fe80::a036:9fff:fe12:3456
  3. C
    fe80::a236:9fff:ffff:3456
  4. D
    fc00::a236:9fff:fe12:3456

Answer

The automatically assigned link-local address is fe80::a236:9fff:fe12:3456.
The correct answer derives the link-local address by taking the MAC address a036.9f12.3456, inserting FFFE in the middle (a036.9fff.fe12.3456), and inverting the 7th bit of the first byte from 0 to 1 (0xA0 binary 10100000 -> 0xA2 binary 10100010). Combining this 64-bit interface identifier with the link-local prefix FE80:: results in fe80::a236:9fff:fe12:3456.

Step-by-Step Solution

1
Split the MAC address into two 24-bit halves.
High-order OUI: a0:36:9f; Low-order NIC ID: 12:34:56.
EUI-64 formatting requires inserting a 16-bit payload into the middle of a standard 48-bit MAC address.
2
Insert the hexadecimal value FFFE between the two halves.
a0:36:9f:ff:fe:12:34:56.
IEEE EUI-64 expansion rule inserts FFFE (11111111 11111110 binary) at the midpoint.
3
Invert the 7th bit (Universal/Local bit) of the first byte.
First byte 0xA0 (10100000 binary) becomes 0xA2 (10100010 binary), yielding interface ID a236:9fff:fe12:3456.
In EUI-64 interface identifiers derived from IEEE MAC addresses, the 7th bit is flipped to indicate universal scope when set to 1.
4
Combine the calculated 64-bit interface ID with the link-local prefix fe80::/10.
fe80::a236:9fff:fe12:3456.
IPv6 link-local autoconfiguration appends the EUI-64 interface ID to fe80::/10.

Key Concept

Generating an IPv6 EUI-64 interface identifier from a 48-bit MAC address requires inserting FFFE into the middle of the MAC address, flipping the 7th bit (Universal/Local bit) of the first octet, and prepending the fe80::/10 prefix for link-local addresses.
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Topics

IPv6 Address Types and EUI-64

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