If you would like Internet services (such as Google Docs or Pixlr) to access your DiskStation, you can specify the public, external hostname and port number at Control Panel > External Access > Advanced.
By default, the hostname is your public IP address (or hostname set in Control Panel > External Access > DDNS), and the port number is 5000 (or 5001 for HTTPS connections). However, if you have another hostname that is not configured at the DDNS page, or you have specified different port numbers on your router when setting up port forwarding rules, you will need to enter the hostname or router port numbers to make sure files you shared to Internet services can be accessed successfully.