diff --git a/lib/formsg-on-cdk-stack.ts b/lib/formsg-on-cdk-stack.ts
index 95cc126..040c8ad 100644
--- a/lib/formsg-on-cdk-stack.ts
+++ b/lib/formsg-on-cdk-stack.ts
@@ -63,7 +63,7 @@ export class FormsgOnCdkStack extends cdk.Stack {
           return new ec2.PrivateSubnet(this, `ddb-subnet-${index}`, {
             availabilityZone,
             vpcId: vpc.vpcId,
-            cidrBlock: `10.1.${(index + 1) * 64}.0/18`,
+            cidrBlock: `10.0.${64 + index}.0/18`,
           })
         })
       },