In iphone, how to check whether internet is there from the app?

When my app starts, I want to check whether the net connection is 开发者_Go百科avaliable or not. And if not I want to show an alert message. How can I check it?

Check out Apple's Reachability example: http://developer.apple.com/iphone/library/samplecode/Reachability/index.html. It will show you everything you need on how to detect the network state of the device.