Public IP connectivity is most appropriate when the source database is external to Google Cloud
and has an externally accessible IPv4 address and TCP port. If the source
database is hosted in another VPC in Google Cloud, then the easiest way to
connect the source database with the Cloud SQL instance is by using VPC Peering.
If your source database is external to Google Cloud, then add the destination database's outgoing
IP address (and port 5432) as an inbound firewall rule on the source network. In
generic terms (your specific network settings may differ), do the following:
Open the source database machine's network firewall rules.
Create an inbound rule.
Set the Rule type to PostgreSQL.
Set the Protocol to TCP.
Set the Port range to 5432.
Set the Source IP address to the destination database's outgoing IP address. For example:
220.127.116.11/32. (The /32 designation in CIDR notation limits the
address range to one address only, the one provided. It's setting the subnet
mask to 255.255.255.255).