You are running out of primary internal IP addresses in a subnet for a custom mode VPC. The subnet has the IP range 10.0.0.0/20, and the IP addresses are primarily used by virtual machines in the proj

Adding a secondary IP range to the subnet allows virtual machines to be configured with alias IP addresses from this new range, effectively providing more available IP addresses without modifying the existing primary range or requiring a new subnet.

Subnet primary IP ranges cannot be changed or expanded after creation; doing so would typically require creating a new subnet and migrating resources, which is disruptive.

Changing the subnet IP range from /20 to /22 would decrease the number of available IP addresses (a /22 is smaller than a /20), making the problem worse.

Converting to IPv6 addresses would provide a vast number of new addresses, but it's a significant network architecture change and does not directly solve the need for more IPv4 addresses for existing VMs in the current setup.