DNS request are usually cached, that’s good as it help to speeds up the lookups within the same host but sometimes we will want to clear the cache so it don’t hold the values that are no longer valid.
In OS X we use terminal to clear DNS cache, but in iOS there is no terminal. You have 2 possibilities to clear dns cache in iOS, first is reboot and second one is switching to flight mode. Because rebooting is taking more time and when you need to clear DNS multiple times is easier and faster to use flight mode.
Airplane Mode completely disables all network connectivity until you turn it off and has the side effect of flushing the DNS cache of the iPhone.
Go to Settings and turn ON Airplane Mode wait 5-10 seconds and then turn it off.
With iOS 7 is possible to switch even faster.
Pull up bottom menu and press on Airplane Mode button, wait 5-10 seconds and then press again to disable it.