NetBIOS over TCP/IP (port 139) is not secured and should be blocked from inbound or outbound access through the firewall to the Internet/DMZ for security reasons. To allow this port through the firewall is to defeat the purpose of having a firewall in the first place. Windows NT uses Port 139 for null sessions and for file sharing; Windows 2000 uses (for SMB) port 445 if port 139 is not enabled.
Hope this provided the information you needed.
This was first published in August 2002