Diagnostic Database Connection

1. ping
ping <database ip address>

2. tnsping
tnsping <net service name>
where the "net service name" must exist in the tnsnames.ora file

3. sqlplus
sqlplus <login>/<password>@<net service name>
where the "net service name" must exist in the tnsnames.ora file

http://www.oracleutilities.com/OSUtil/ping.html