A network administrator is allocated the IPv6 prefix block 2001:\text{db8}:5678:\text{a000… — Cisco CCNA

A network administrator is allocated the IPv6 prefix block $2001:\text{db8}:5678:\text{a000}::/52$ for an enterprise deployment. Which two IPv6 addresses belong to valid subnets within this assigned prefix block? (Select two.)

2001:db8:5678:a4f0:1::1/64Answer
2001:db8:5678:ab00::1/64Answer
C
2001:db8:5678:b000::1/64
D
2001:db8:5678:a00::1/64

Answer

The correct IPv6 addresses are 2001:db8:5678:a4f0:1::1/64 and 2001:db8:5678:ab00::1/64 because their fourth hextet values (a4f0 and ab00) lie within the range a000 to afff specified by the /52 prefix boundary.

The correct options are the addresses featuring 'a4f0' and 'ab00' as their fourth hextet. A /52 prefix locks the first 52 bits of the IPv6 address. The first three 16-bit hextets supply 48 bits, leaving 4 bits to lock the first hex digit of the fourth hextet. Since 'a' represents binary 1010 across bits 49–52, any valid address under this prefix must have a fourth hextet starting with 'a' (ranging from a000 to afff).

Step-by-Step Solution

Analyze the assigned IPv6 prefix length and identify hex nibble boundaries.

The prefix

2001:\text{db8}:5678:\text{a000}::/52

has

52

fixed network bits. Each hex character (nibble) represents

4

bits. The first three hextets (

2001:\text{db8}:5678

) account for

48

bits (

3 \times 16

). The remaining

4

network bits (

52 - 48 = 4

) lock the first nibble of the fourth hextet.

Determining where the prefix boundary falls inside the fourth hextet allows precise identification of valid subnet ranges.

Calculate the valid range of values for the fourth hextet.

The hex character

\text{'a'}

corresponds to binary

1010

. Since the first

4

bits of the fourth hextet must remain

1010

, the fourth hextet can range from

\text{a000}

(

1010\ 0000\ 0000\ 0000_2

) to

\text{afff}

(

1010\ 1111\ 1111\ 1111_2

Any valid subnet address must have a fourth hextet between a000 and afff.

Evaluate each candidate address against the calculated fourth hextet range.

The addresses containing 'a4f0' and 'ab00' in their fourth hextet are within the range a000-afff. The address with 'b000' starts the next block (

2001:\text{db8}:5678:\text{b000}::/52

), and 'a00' expands to '0a00', which falls below 'a000'.

Only addresses matching the leading nibble 'a' in the fourth hextet belong to the designated /52 block.

Key Concept

IPv6 Prefix Length Boundaries and Hexadecimal Subnet Range Calculation

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