Access Control List(ACL) is a fine-grained security mechanism. It is a list of access control entries to restrict the hosts that are allowed to connect to the Oracle database.

Users or roles are called principals.
Operations are called privileges.

Following are the steps for Create ACLs for public or HR schema:

Create the ACL

BEGIN
DBMS_NETWORK_ACL_ADMIN.CREATE_ACL (
acl => 'hr_utl_http.xml',
description => 'Allow mail',
principal => 'PUBLIC',  --- you can assign PUBLIC or user like 'HR'
is_grant => TRUE,
privilege => 'connect',
start_date => SYSTIMESTAMP,
end_date => NULL);
COMMIT;
END;
/

2. Assign ACL to network.

BEGIN
DBMS_NETWORK_ACL_ADMIN.ASSIGN_ACL (
acl => 'hr_utl_http.xml',
host => '192.168.63.63', -- specify the host IP address 
lower_port => 34,       -- specify the port range lower value (* for all)
upper_port => 63);      -- specify the port range higher value
COMMIT;
END;
/

3. Grant privilege to user or Public

-- Grant connect permission 
BEGIN
DBMS_NETWORK_ACL_ADMIN.ADD_PRIVILEGE(
acl => 'hr_utl_http.xml',
principal => 'PUBLIC',  --if you want user like 'HR' 
is_grant => true,
privilege => 'connect');
COMMIT;
END;
/

-- grant resolve permission
BEGIN
DBMS_NETWORK_ACL_ADMIN.ADD_PRIVILEGE(
acl => 'hr_utl_http.xml',
principal => 'PUBLIC',  --if you want user like 'HR' 
is_grant => true,
privilege => 'resolve');
COMMIT;
END;
/

4. Verified the ACL is created.

SELECT * FROM DBA_NETWORK_ACLS;
SELECT * FROM DBA_NETWORK_ACL_PRIVILEGES;

Rollback or Remove the ACL permission.

Remove the created permission or ACL rule from the Oracle Database.

--Un-assign network from ACL:
exec DBMS_NETWORK_ACL_ADMIN.UNASSIGN_ACL(acl =>'hr_utl_mail.xml',lower_port =>'*',upper_port =>25);

-- Remove privilege from an user:
exec DBMS_NETWORK_ACL_ADMIN.DELETE_PRIVILEGE(acl =>'hr_utl_mail.xml',principal =>'PUBLIC',is_grant =>TRUE,privilege =>'connect');
exec DBMS_NETWORK_ACL_ADMIN.DELETE_PRIVILEGE(acl =>'hr_utl_mail.xml',principal =>'PUBLIC',is_grant =>TRUE,privilege =>'resolve');

-- Drop ACL:
exec DBMS_NETWORK_ACL_ADMIN.DROP_ACL (acl => 'hr_utl_mail.xml' );