i don't know what to search in google for this topic , so plz at least tell me what to search for, if not give the answer.. my queston is..

if some how i know the ip addersse of a computer on net at sometime which is having dynamic address.what should i do then to constantly moniter its changing ip adderesss after sometime or day.? or say want to be in constant knowledge of its ip regularly after that even if it changes regularly .. how can i do that??