Event 4625 Audit failures in logs while RDP is open only to my own IP

This is our windows 2008 R2 server with all the updates installed. I have limited RDP access to my own IPs only (Windows firewall rules). When I try to RDP from any other IP, don't even get username/pass screen (means rules have been set correctly). Also only RDP and http and https ports are open. But I see many login attempts like this in security log:

An account failed to log on.

Subject:
    Security ID:        NULL SID
    Account Name:       -
    Account Domain:     -
    Logon ID:       0x0

Logon Type:         3

Account For Which Logon Failed:
    Security ID:        NULL SID
    Account Name:       Administrator
    Account Domain:     CLUSTER02

Failure Information:
    Failure Reason:     Unknown user name or bad password.
    Status:         0xc000006d
    Sub Status:     0xc000006a

Process Information:
    Caller Process ID:  0x0
    Caller Process Name:    -

Network Information:
    Workstation Name:   CLUSTER02
    Source Network Address: 113.177.2.43
    Source Port:        2176

Detailed Authentication Information:
    Logon Process:      NtLmSsp 
    Authentication Package: NTLM
    Transited Services: -
    Package Name (NTLM only):   -
    Key Length:     0

This event is generated when a logon request fails. It is generated on the computer where access was attempted.

The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.

The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network).

The Process Information fields indicate which account and process on the system requested the logon.

The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.

The authentication information fields provide detailed information about this specific logon request.
    - Transited services indicate which intermediate services have participated in this logon request.
    - Package name indicates which sub-protocol was used among the NTLM protocols.
    - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.

how can I stop these attacks? Thanks in advance for your help.