Reserved CIDR range in a Subnet

Question

Customer wants to reserve a CIDR range from the CIDR of a subnet. They want to keep the CIDR for private use, and the IPs in the CIDR can't be auto assigned when running instances with auto assigned private IPs.

An example, there is a subnet with CIDR 10.85.0.0/18, the customer wants keep 10.85.1.0/24 for other use, that means when running run-instances in this subnet, the EC2 instances shouldn't auto get a private IP from the reserved CIDR 10.85.1.0/24.

Do we have this feature or workaround for this requirement?

Accepted Answer

Although it's a little ugly, one option would be to create ENIs with specific IP addresses that should be in their reserved range. They can automate this using the "create-network-interface" EC2 API so it wouldn't be difficult. They should tag the ENIs that are "holding" IP addresses so that it is obvious what they are doing.

Reserved CIDR range in a Subnet

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