We recently consolidated websites/servers hosted on AWS EC2.
On ServerA, we released a couple of elastic IP address. Using EC2 –> Networking –> Manage Private IP Addresses, I unassigned those orphaned private IPs.
After reboot, we found that ServerA can no longer connect to another SQL server.
It turns out that I forgot one thing. We used to have those orphaned private IPs as static IPs, instead of DHCP on the ServerA OS network setting. Thus, I removed those obsolete private IPs from network adapter setting IPv4.
The lesson is that any extra obsolete private IPs on the server network setting could lead to SQL connection issues.