ORA-12514: TNS:listener does not currently know of service requested in connect descriptor on linux

I have configured oracle 12c on Rhel 7. When I try to connect database using following command it gets connected with no issues: sqlplus system/Forest123@orcl I'm facing problem when I try to put the connection string with hostname and port as shown below: sqlplus system/Forest123@localhost:1521/orcl Error occurred is > ORA-12514: TNS:listener does not currently know of service requested in connect descriptor I have gone through the questions asked before, but none of them helped me solve this problem. Following are my tnsnames.ora, sqlnet.ora and listener.ora files. **tnsnames.ora** ORCL = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = oracle_12C.localdomain)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = orcl.localdomain) ) ) **listener.ora** LISTENER = (DESCRIPTION_LIST = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = oracle_12C.localdomain)(PORT = 1521)) (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521)) ) ) **sqlnet.ora** NAMES.DIRECTORY_PATH= (TNSNAMES, EZCONNECT) **hosts file** 127.0.0.1 localhost 127.0.0.1 localhost.localdomain 127.0.0.1 localhost4 127.0.0.1 localhost4.localdomain4 ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6 ::127.0.0.1 localhost.localdomain localhost 172.31.15.86 oracle_12C.localdomain oracle_12C