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Difficulty: Very hardIPv6 Addressing and Prefixes

An organization is assigned the IPv6 global routing prefix 2001:db8:abc0::/482001:\text{db8}:\text{abc0}::/48. A network engineer needs to configure the 16th subnet (subnet index 15 in hexadecimal count starting from 0) using standard /64/64 subnets. The router interface on this subnet is configured to dynamically derive its 64-bit interface identifier using EUI-64 based on its MAC address of 70-69-79-A1-B2-C3\text{70-69-79-A1-B2-C3}. Which fully formed and compressed IPv6 global unicast address will be assigned to this interface?

  1. 2001:db8:abc0:f:7269:79ff:fea1:b2c3Answer
  2. B
    2001:db8:abc0:f:7069:79ff:fea1:b2c3
  3. C
    2001:db8:abc0:f:7269:79a1:b2c3
  4. D
    2001:db8:abc0:15:7269:79ff:fea1:b2c3

Answer

The correct IPv6 address is 2001:db8:abc0:f:7269:79ff:fea1:b2c3.
The correct answer properly converts the 16th subnet index (15) to hexadecimal 'f', inserts the 'FFFE' pattern into the MAC address, and flips the 7th bit of the first byte (from 0x70 to 0x72) to form the valid EUI-64 interface identifier.

Step-by-Step Solution

1
Determine the Subnet ID
The subnet ID is 0x000F (compressed to 'f').
Starting from subnet index 0 (2001:db8:abc0:0::/642001:\text{db8}:\text{abc0}:0::/64), the 16th subnet corresponds to index 15, which in hexadecimal is 0x000F.
2
Insert FFFE into the MAC address
7069:79FF:FEA1:B2C3
EUI-64 construction requires splitting the 48-bit MAC address (70-69-79-A1-B2-C3) into two 24-bit halves and inserting 0xFFFE between them.
3
Invert the Universal/Local (7th) bit of the first byte
First byte changes from 0x70 to 0x72.
The first byte 0x70 in binary is 0111 0000. Inverting the 7th bit (bit position 7 from MSB) yields 0111 0010, which is 0x72 in hexadecimal. Thus, the interface ID becomes 7269:79ff:fea1:b2c3.
4
Combine the network prefix, subnet ID, and interface ID
2001:db8:abc0:f:7269:79ff:fea1:b2c3
Combining 2001:db8:abc0::2001:\text{db8}:\text{abc0}:: + f::f:: + 7269:79ff:fea1:b2c37269:79\text{ff}:\text{fea1}:b2c3 produces the complete IPv6 address.

Key Concept

EUI-64 Interface Identifier Construction and Hexadecimal IPv6 Subnetting
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Topics

IPv6 Addressing and Prefixes

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