1. A method implemented in a computing device, the method comprising: storing, in the computing device, an identifier of a geographical region having a predefined geographical boundary; storing, in the computing device, a set of cell identifiers, each of the cell identifiers identifying a cell that is determined to be within the predefined geographical boundary; wherein each respective cell identified by a corresponding cell identifier in the set of cell identifiers is a square area in a longitude latitude space of locations on the earth; receiving, from a mobile device, a location of the mobile device; converting, by the computing device, a set of coordinates of the location of the mobile device to a first cell identifier; and determining, by the computing device, whether or not the location of the mobile device is within the geographical region based on searching the set of cell identifiers to determine whether or not the set has the first cell identifier.