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Difficulty: MediumIPv6 Addressing and Prefixes

A network administrator inspects a Cisco router interface configured with dynamic IPv6 address generation and observes the assigned global unicast address `2001:db8:a1b2:c3d4:0211:22ff:fe33:4455/64`. Based on IEEE EUI-64 specification rules, which MAC address was assigned to the physical interface?

  1. 0011.2233.4455Answer
  2. B
    0211.2233.4455
  3. C
    0011.22ff.4455
  4. D
    0211.22ff.fe33

Answer

The MAC address assigned to the physical interface is 0011.2233.4455.
To derive the original MAC address from a Modified EUI-64 IPv6 interface ID (`0211:22ff:fe33:4455`), remove the inserted `FFFE` hex value from the center to yield six bytes (`02:11:22:33:44:55`) and invert the 7th bit of the first byte (`02` hex = `0000 0010` binary -> inverted to `0000 0000` binary = `00` hex). This reveals the original MAC address `0011.2233.4455`.

Step-by-Step Solution

1
Identify the 64-bit Interface Identifier (Interface ID) from the IPv6 address.
The Interface ID is `0211:22ff:fe33:4455`.
In a /64 prefix, the last 64 bits represent the host portion (Interface ID).
2
Remove the 16-bit `ff:fe` sequence inserted into the middle of the MAC address during EUI-64 generation.
Removing `ff:fe` from `0211:22ff:fe33:4455` leaves 6 bytes: `02-11-22-33-44-55`.
EUI-64 creation inserts `FFFE` between the OUI (first 24 bits) and NIC specific bits (last 24 bits).
3
Invert the 7th bit (Universal/Local bit) of the first byte.
The first byte `02` in binary is `0000 0010`. Inverting the 7th bit yields `0000 0000`, which is `00` in hex. Combining all bytes yields `0011.2233.4455`.
The 7th bit is flipped from 0 to 1 when constructing Modified EUI-64, so it must be flipped back from 1 to 0 to restore the original MAC address.

Key Concept

Modified EUI-64 MAC-to-IPv6 Address Derivation
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Topics

IPv6 Addressing and Prefixes

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