https://docs.aws.amazon.com/whitepapers/latest/sddc-deployment-and-best-practices/deploying-vmware-cloud-on-aws-sddc.htmlThis must be a RFC1918 private address space (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16) with CIDR block sizes of /16, /20, or /23. The management CIDR block cannot be changed after the SDDC is deployed. Choose a range of IP addresses that does not overlap with the AWS subnet you are connecting to. If you plan to connect the SDDC to an on-premises DC or another environment, the IP subnet must be unique within your enterprise network infrastructure. Choose a CIDR that will give you future scalability.