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Difficulty: MediumIPv6 Addressing and Prefixes

A network administrator is assigned the IPv6 prefix block `2001:db8:cafe:a000::/56` for a branch office. The administrator must divide this allocation into multiple `/64` subnets for individual client VLANs. Which prefix represents the last `/64` subnet available within this assigned block?

  1. 2001:db8:cafe:a0ff::/64Answer
  2. B
    2001:db8:cafe:a0fe::/64
  3. C
    2001:db8:cafe:a00f::/64
  4. D
    2001:db8:cafe:a100::/64

Answer

The prefix 2001:db8:cafe:a0ff::/64 represents the last /64 subnet available within the assigned /56 block.
The prefix 2001:db8:cafe:a0ff::/64 correctly identifies the last /64 subnet within the 2001:db8:cafe:a000::/56 assignment. A /56 prefix allocates 8 bits (bits 57 through 64) for creating /64 subnets, which correspond to the last two hexadecimal digits of the fourth hextet. The range of the fourth hextet for these subnets is from a000 to a0ff.

Step-by-Step Solution

1
Determine the number of subnet bits available between the assigned prefix length (/56) and the target prefix length (/64).
64 - 56 = 8 subnet bits, which equal 2 hexadecimal digits.
Each hexadecimal character in an IPv6 address represents 4 bits (1 nibble).
2
Identify which hextet and bit positions contain these subnet bits.
The first 48 bits cover the first three hextets (2001:db8:cafe:). The assigned /56 prefix includes the first 8 bits of the fourth hextet ('a0'). The subnet bits occupy the remaining 8 bits of the fourth hextet.
The fourth hextet spans bits 49 through 64.
3
Calculate the maximum value for the 8 subnet bits.
8 bits of all binary 1s equal hex 'ff'. The fourth hextet ranges from 'a000' (first subnet) to 'a0ff' (last subnet).
Hexadecimal 'a0' followed by 'ff' forms the last valid /64 prefix.

Key Concept

IPv6 Subnetting and Prefix Length Calculations
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Topics

IPv6 Addressing and Prefixes

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