There are a couple of possible reasons for this error:
- DNS resolution problems (try
nslookup <hostname>
to verify DNS resolution is working) - the listener might be running on another port (standard is 1521, check with your DBA if necessary)
- there might be a firewall in between that blocks you from accessing the database (does connecting locally on the server work? Can you connect from another machine?)
- the listener on the database might not be running (try connecting from your machine with
sqlplus
)