move splitting of IPv6 CIDRs to ip-addresses

packages/aws-cdk-lib/aws-ec2/lib/ip-addresses.ts

@@ -138,7 +138,7 @@ export interface AllocateCidrRequest {
 }
 
 /**
- * Request for subnet IPv6 CIDRs to be allocated for a VPC
+ * Request for subnet IPv6 CIDRs to be allocated for a VPC.
  */
 export interface AllocateIpv6CidrRequest {
   /**
@@ -152,6 +152,27 @@ export interface AllocateIpv6CidrRequest {
   readonly ipv6Cidrs: string[];
 }
 
+/**
+ * Request for IPv6 CIDR block to be split up.
+ */
+export interface CreateIpv6CidrBlocksRequest {
+  /**
+   * The IPv6 CIDR block string representation.
+   */
+  readonly ipv6SelectedCidr: string,
+
+  /**
+   * The number of subnets to assign CIDRs to.
+   */
+  readonly subnetCount: number,
+
+  /**
+   * Size of the covered bits in the CIDR.
+   * @default - 128 - 64 = /64 CIDR.
+   */
+  readonly sizeMask?: string,
+}
+
 /**
  * CIDR Allocated Subnets
  */
@@ -421,6 +442,13 @@ export interface IIpv6Addresses {
    * Note this is specific to the IPv6 CIDR.
    */
   allocateSubnetsIpv6Cidr(input: AllocateIpv6CidrRequest): SubnetIpamOptions;
+
+  /**
+   * Split IPv6 CIDR block up for subnets.
+   *
+   * Note this is specific to the IPv6 CIDR.
+   */
+  createIpv6CidrBlocks(input: CreateIpv6CidrBlocksRequest): string[];
 }
 
 /**
@@ -461,4 +489,17 @@ class AmazonProvided implements IIpv6Addresses {
       allocatedSubnets: allocatedSubnets,
     };
   }
+
+  /**
+   * Split IPv6 CIDR block up for subnets.
+   *
+   * Called by VPC when creating subnets with IPv6 enabled.
+   *
+   * Note this is specific to the IPv6 CIDR.
+   */
+  createIpv6CidrBlocks(input: CreateIpv6CidrBlocksRequest): string[] {
+    const sizeMask = input.sizeMask ?? '64'; // 128 - 64
+
+    return Fn.cidr(input.ipv6SelectedCidr, input.subnetCount, sizeMask);
+  }
 }

packages/aws-cdk-lib/aws-ec2/lib/vpc.ts

@@ -1784,7 +1784,10 @@ export class Vpc extends VpcBase {
         throw new Error('No IPv6 IpAddresses were found');
       }
       // create the IPv6 CIDR blocks
-      const subnetIpv6Cidrs = Fn.cidr(this.ipv6SelectedCidr, allocatedSubnets.length, (128 - 64).toString());
+      const subnetIpv6Cidrs = this.ipv6Addresses.createIpv6CidrBlocks({
+        ipv6SelectedCidr: this.ipv6SelectedCidr,
+        subnetCount: allocatedSubnets.length,
+      });
 
       // copy the list of allocated subnets while assigning the IPv6 CIDR
       const allocatedSubnetsIpv6 = this.ipv6Addresses.allocateSubnetsIpv6Cidr({