A company wants to associate an IPv6 CIDR block with its VPC and specifically wants Amazon to allocate the block from Amazon's own IPv6 address pool rather than bringing their own range. Which statement correctly describes what happens?
- The company chooses any /56 IPv6 range they like, and AWS reserves it for their exclusive use
- AWS assigns a fixed-size IPv6 CIDR block (a /56) from its own pool, and the company cannot choose the specific range of addresses themselves
- AWS requires the company to first own a public IPv4 range before it will allocate any IPv6 block
- IPv6 CIDR blocks can only be added to a VPC at creation time and can never be added to an existing VPC later
Why B? And why not the others?
Correct answer: B. AWS assigns a fixed-size IPv6 CIDR block (a /56) from its own pool, and the company cannot choose the specific range of addresses themselves
When a VPC requests an Amazon-provided IPv6 CIDR block, AWS documentation gives the example of Amazon assigning a /56 block such as 2001:db8:1234:1a00::/56 and states directly, 'You cannot choose the range of IP addresses yourself' — the customer only chooses to request an Amazon-provided block, not which specific addresses it contains. Option A is wrong because self-selection of the specific range is exactly what AWS says is not possible with Amazon-provided blocks; choosing your own range requires bringing your own IPv6 range (BYOIP) instead. Option C is wrong because Amazon-provided IPv6 allocation has no prerequisite tied to owning a public IPv4 range. Option D is wrong because IPv6 CIDR blocks, like additional IPv4 CIDR blocks, can be associated with an existing VPC after creation, not only at creation time.
Source: AWS VPC docs: IP addressing for your VPCs and subnets — VPC CIDR blocks