PRTG Manual: Login
Once the PRTG core server is installed, you can log in to the web interface. In your browser, load the IP address or DNS name of the computer PRTG is installed on and log in using the Default Login button.
You can look up and change PRTG's web server settings at any time using the PRTG Server Administrator Windows application on the system where the PRTG core server is installed on. Especially when accessing PRTG from the internet you should use an SSL encrypted connection. You can easily switch to SSL using the Yes, switch to SSL button shown on the welcome screen.
In a web browser window, please enter the IP address or URL of the system PRTG is installed on. When using a cluster, please connect to the primary master node. You can also double click on the PRTG Network Monitor icon on the desktop of the system PRTG is installed on.
Note: If you run PRTG on localhost, please do not use the DNS name to log in to the web server, as this may considerably slow down PRTG's web interface. Please use your local IP address or instead.

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來自: http://n.yam.com/help/login.htm#login