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Difficulty: HardIPv6 Addressing and Prefixes

An organization is assigned the IPv6 address block 2001:db8:1111:2000::/522001:\text{db8}:1111:2000::/52. A network engineer needs to configure a router interface within a new /64 subnet that has a 12-bit subnet ID extension of 0xA400x\text{A40} added to the base /52 prefix. The router interface uses EUI-64 to generate its interface identifier automatically based on its MAC address `0200.4C12.3456`. Which IPv6 Global Unicast Address will be dynamically configured on this interface?

  1. 2001:db8:1111:2a40::4cff:fe12:3456Answer
  2. B
    2001:db8:1111:2a40:200:4cff:fe12:3456
  3. C
    2001:db8:1111:a40::4cff:fe12:3456
  4. D
    2001:db8:1111:2a40:400:4cff:fe12:3456

Answer

2001:db8:1111:2a40::4cff:fe12:3456
The subnet prefix is derived by adding the subnet ID extension 0xA400x\text{A40} to the base prefix 4th hextet (20002000), producing 2A402\text{A}40. For EUI-64 generation from MAC `0200.4C12.3456`, `FFFE` is inserted into the middle (`0200.4CFF.FE12.3456`), and the 7th bit of the first byte (`0x02` = `0000 0010`) is inverted to `0` (`0x00` = `0000 0000`), yielding `0000:4cff:fe12:3456`. Combining these results in `2001:db8:1111:2a40::4cff:fe12:3456`.

Step-by-Step Solution

1
Calculate the /64 subnet prefix from the assigned /52 block and subnet offset
Subnet prefix is 2001:db8:1111:2a40::/64
The base /52 prefix ends at the 4th hex digit of the 4th hextet (20002000). Adding the 12-bit hex extension 0xA400x\text{A40} yields 2000+A40=2A402000 + \text{A40} = 2\text{A}40 in hexadecimal for the 4th hextet.
2
Split the MAC address 0200.4C12.3456 into two 24-bit halves and insert FFFE
Intermediate identifier is 0200:4CFF:FE12:3456
EUI-64 requires inserting the hex value FFFE into the middle of the 48-bit MAC address.
3
Invert the 7th bit (Universal/Local bit) of the first byte
First byte 0x02 (binary 0000 0010) becomes 0x00 (binary 0000 0000)
In EUI-64 generation, flipping the 7th bit changes a 1 to a 0 (or 0 to 1), transforming 0200:4CFF:FE12:3456 into 0000:4CFF:FE12:3456 (compressed as ::4cff:fe12:3456).
4
Combine the /64 prefix with the EUI-64 interface identifier
2001:db8:1111:2a40::4cff:fe12:3456
Merging 2001:db8:1111:2a40::/64 and 0000:4cff:fe12:3456 gives the final global unicast address.

Key Concept

IPv6 EUI-64 Interface Identifier Generation and Subnet Prefix Calculation
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Topics

IPv6 Addressing and Prefixes

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