I just installed 12c on a CentOS server and it is working fine.  I can connect locally with sqlplus with / as sysdba and sys@orcl as sysdba. 

But when I try to set up a connection in SQL Developer 17.4.0.355, on my mac, in the connection setup dialog and click the test I get:

Status: Failure- Test failed: IO Error : The network adapter could not establish the connection

I feel like this is a problem with the MAC and not the DB, any Ideas?

Larry