Firebird on Windows has the following switches to explicitly enable connection protocols:
-i enables INET protocol, it is used when the connection string looks like localhost:Disk\Path\database.fdb or server_name:
C:\HQbird\Firebird30>firebird -a C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey localhost:o:\OLTP30.V20190415.FDB Database: localhost:o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit; C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey xnet://o:\OLTP30.V20190415.FDB Database: xnet://o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit; C:\HQbird\Firebird30>firebird -a -x C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey localhost:o:\OLTP30.V20190415.FDB Statement failed, SQLSTATE = 08006 Unable to complete network request to host "localhost". -Failed to establish a connection. Use CONNECT or CREATE DATABASE to specify a database SQL> exit; C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey xnet://o:\OLTP30.V20190415.FDB Database: xnet://o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit; C:\HQbird\Firebird30>firebird -a -i C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey localhost:o:\OLTP30.V20190415.FDB Database: localhost:o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit; C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey xnet://o:\OLTP30.V20190415.FDB Statement failed, SQLSTATE = 08006 Unable to complete network request to host "xnet://Global\FIREBIRDHQBIRD". Use CONNECT or CREATE DATABASE to specify a database SQL> exit; C:\HQbird\Firebird30>firebird -a -i -x C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey localhost:o:\OLTP30.V20190415.FDB Database: localhost:o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit; C:\HQbird\Firebird30>isql -user SYSDBA -pass masterkey xnet://o:\OLTP30.V20190415.FDB Database: xnet://o:\OLTP30.V20190415.FDB, User: SYSDBA SQL> exit;