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Difficulty: HardIPv6 Addressing and Deployment

A network engineer is deploying IPv6 across a corporate branch office after receiving a /56/56 IPv6 prefix delegation of 2001:db8:1234:5600::/562001:\text{db8}:1234:5600::/56 from the upstream Internet Service Provider (ISP). Enterprise standards require that all internal local area networks (LANs) be provisioned using standard /64/64 subnets to support Stateless Address Autoconfiguration (SLAAC). How many total /64/64 subnets can the engineer allocate from this delegated prefix, and what is the RFC 5952 compliant IPv6 prefix for the final available subnet?

  1. 256 subnets, with the final subnet prefix designated as 2001:db8:1234:56ff::/64Answer
  2. B
    254 subnets, with the final subnet prefix designated as 2001:db8:1234:56fe::/64
  3. C
    16 subnets, with the final subnet prefix designated as 2001:db8:1234:56f0::/64
  4. D
    256 subnets, with the final subnet prefix designated as 2001:db8:1234:5600::ff::/64

Answer

The correct subnet count is 256 subnets, and the final subnet prefix is 2001:db8:1234:56ff::/64.
Subtracting the ISP delegation prefix length (/56) from the standard host SLAAC prefix length (/64) yields 8 bits for internal subnet division. Calculating 282^8 produces 256 total subnets. The first 56 bits (2001:db8:1234:562001:\text{db8}:1234:56) remain fixed, while the remaining 8 bits of the fourth hextet range from 0x00 to 0xFF hex. Thus, the last valid /64 subnet prefix is 2001:db8:1234:56ff::/64.

Step-by-Step Solution

1
Calculate the number of bits allocated for subnetting.
Subnet bits = 64−56=864 - 56 = 8 bits.
The delegated prefix is /56 and target LAN subnets require a /64 prefix length for SLAAC.
2
Determine total usable subnets from the 8 subnet bits.
28=2562^8 = 256 subnets.
In IPv6 subnetting, there are no reserved all-zeros network or all-ones broadcast subnet restrictions like in IPv4; all combinations are usable.
3
Calculate the fourth hextet range in hexadecimal.
The fourth hextet is 16 bits long: bits 48-55 are fixed as 0x56, and bits 56-63 range from 0x00 to 0xFF.
Starting at 0x5600, incrementing the 8-bit subnet ID from 0x00 to 0xFF results in a fourth hextet range of 5600 to 56FF.
4
Format the final subnet prefix according to RFC 5952 compression guidelines.
2001:db8:1234:56ff::/64
Trailing zero hextets in the host portion are replaced by a single double-colon (::).

Key Concept

IPv6 Prefix Subnetting and RFC 5952 Address Formatting
Estimated Time:2m 0s
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Topics

IPv6 Addressing and Deployment

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