OnChange is a TNotifyEvent property that specifies the event handler for changes to network address properties. OnChange is triggered from both the OnNetAddressChange and OnNetMaskChange event handlers following a change to the values of either property. 

Use OnChange to notify the application of changes in the TIdNetworkCalculator network address properties. Assign an OnChange event handler to respond to the event notification.

OnNetAddressChange OnNetMaskChange