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Hi,
When you create a VPC, you must specify an IPv4 CIDR block. The allowed block size is between /16 and /28 netmasks. After the VPC is created, you can associate additional IPv4 CIDR blocks with the VPC. You can have up to five CIDR blocks by default, but this limit is adjustable, see Add an IPv4 CIDR block to your VPC for more information.
Best practices is to specify a CIDR block from the private IPv4 address ranges documented in RFC1918. It is possible to create a VPC with a publicly routable CIDR block, but I would recommend working with your account Solutions Architect or open a support ticket if you want to do that.
I imagine if you received a "conflict" error message when trying to add additional CIDR blocks, you incountered some of the IPv4 CIDR block association restrictions documented here: IPv4 CIDR block association restrictions. You should be to determine the conflict from that information and resolve your issue.
Hope that helped.
Hello,
It's allowed to optionally associate additional IPv4 CIDR blocks along with the primary CIDR blocks.
Could you please provide the CIDR ranges that you specified during the VPC creation process?
When you create a VPC, you must specify an IPv4 CIDR block for the VPC. The allowed block size is between a /16 netmask (65,536 IP addresses) and /28 netmask (16 IP addresses).
The following documentation provides an overview of permitted and restricted VPC CIDR block associations. https://docs.aws.amazon.com/vpc/latest/userguide/vpc-cidr-blocks.html#add-cidr-block-restrictions
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Specifically, from the link, make sure that you are not trying to mix 10.x, 172.16.x, and 192.168.x addresses in a VPC. If you have to mix, you can 100.64.0.0/10 to any of the RFC1918 spaces.