Steps to troubleshoot Oracle connectivity:

To troubleshoot & diagnose Oracle connectivity problems, follow this checklist:
 

1. Check with "ping" using the IP address ("ping 198.11.34.43"
2. Check with "ping" using DNS name (e.g. "tnsping uranus")
3. Try telnet to the IP on port 1521 (telnet 1.2.3.4 1521)
4. Check with "tnsping" using TNS service name
5. Invoke SQL*Plus from the OS command line "sqlplus fred@flintstone". If this fails, check to ensure that your listener is the flintstone service defined.
6. Sign-on to SQL*Plus and connect with the TNS name ("connect fred/flintstome@service_name"   
7. Within SQL*Plus, try a select from table@remote_db_lin