Yes, when you start in the splash screen you see in the bottom the run offline button.
In version 2.5.6 you do not need internet unless you want to use the multi-developer functionality.
In 3.0 you can indeed run in offline mode as Ronald already mentioned, but you will miss a lot of the good stuff.
If you are behind a proxy server, you will need to set the global proxy settings and THEN start the Modeler.
Thanks for your prompt replies. 1. I am running the trial version of the Mendix and it does not show that 'run offline' button on the splash screen. So I am not sure whether it's because it's a trial or there is something I am missing. 2. When trying to run pizza-mario project(example), the 'open browser' toolbar icon is disabled, so I can't run on the browser. And when I click on run it gives an error - 'No trial slot available. Please check your subscriptions'. How do I solve this error or enable 'open browser' button?