Use LDAP for name look up with Oracle Instant Client
Place the sqlnet.ora and ldap.ora files in your $ORACLE_ADMIN directory and make sure to set the environment variables. I made mine the TNS_ADMIN directory

(Linux) my .bashrc

export TNS_ADMIN=/opt/oracle/instantclient_12_1/network/admin
export ORACLE_ADMIN=${TNS_ADMIN}

[ku14lts:/opt/oracle/instantclient_12_1/network/admin]
$ ls
ldap.ora  sqlnet.ora  tnsnames.ora


And I was able to connect to a DB not in my tnsnames.ora