Let's assume you've started your Oracle database properly. It's mounted and opened. The LISTENER process is running. You use ps -ef | grep ora command to verify pmon and other processes. You use tnsping to verify if the service is accessible. Everything seems to be ok. However, when you run SQL Developer on your local machine (not on the server), you get an error message which says that the network adapter could not establish the connection - it happened to me while running Oracle 12c on one of my virtual machines. When I was trying to connect to the database from my laptop - I couldn't.
I launched Wireshark and set the filter to display only packets related to the communication between the server and my laptop. I quickly got an answer to the problem - I found that every time I was trying to connect to the server, it would reject each and every attempt. I got ICMP Type 3 Code 10 - Communication with Destination Host is Administratively Prohibited - Firewall, which was running on Oracle Linux 7, was blocking the communication. To simply open port number 1521 I used the following command:
After changing firewall configuration I was able to connect to my Oracle database via SQL Developer.