Assign is an overridden procedure inherited from TPersistent that copies the properties or other attributes of Source to the current instance. 

Assign calls SetAll to update the Byte values of the IP address or network mask. Assigns triggers the OnChange event handler to signify that a change has occurred in TIpProperty values. Finally, Assign calls the inherited Assign method to handle assignment of properties from the class of the Source parameter.

SetAll 

OnChange

ANetCalc.NetworkAddress.Assign(AIPProperty);