The Base URL field at the top of the Selenium IDE window is very useful for allowing test cases to be run across different domains. Suppose that a site named http://www.seleniummaster.com had a dev site named http://dev.seleniummaster.com Any test cases for these sites that begin with an open statement should specify a relative URL as the argument to open rather than an absolute URL (one starting with a protocol such as http: or https:).
Selenium IDE will then create an absolute URL by appending the open command’s argument onto the end of the value of Base URL. For example, the test case below would be run against http://www.seleniummaster.com/about.html.
This same test case with a modified Base URL setting would be run against http://dev.seleniummaster.com/about.html.