Io exception: The Network Adapter could not establish the connection

For 2 days when I wanted to connect to target DB from Cloud control 12c, the console was throwing this error:

Io exception: The Network Adapter could not establish the connection.

I did some checking on cloud and target DB that listeners and agents was in a correct state but Metalink gave some advice to me :

- check if there is firewall
- check the host name and port number
- check the host, port in the target.xml
- check if the sysman, dbsnmp is locked

After we checked firewall roles with network administrator we found problem on the firewall because of when you register a RAC DB on the Cloud therefore you have to open public ips and SCAN ips policy’s on the firewalls and we didn’t open them. After that the problem resolved.