1. A computer-implemented method executed using one or more computers, the method comprising: receiving, by the one or more computers and from a user device, a targeting request to target a piece of digital content to users in a target geographic region, wherein the targeting request includes one or more Internet Protocol blocks corresponding to the target geographic region; obtaining, by the one or more computers, a targeting accuracy for the target geographic region, the targeting accuracy for a geographic region representing an accuracy with which a data processing system can determine whether a computing device is located in the geographic region; identifying, by the one or more computers, a plurality of groups of candidate geographic regions that each include: (i) the target geographic region, and (ii) at least one other geographic region that is determined to be similar to the target geographic region based at least on an analysis of data associated with the Internet Protocol blocks corresponding to the target region; selecting, by the one or more computers, a particular group of geographic regions from among the plurality of groups of candidate geographic regions that has a combined targeting accuracy greater than the targeting accuracy for the target geographic region; and providing, by the one or more computer and for presentation on the user device, a targeting interface that: (i) displays data indicating the particular group of geographic regions that was selected from the plurality of groups of candidate geographic regions, and (ii) adds the at least one other geographic region, that is included in the particular group of geographic regions, to the target geographic region to which the piece of digital content is to be targeted, upon selection of the particular group of geographic regions by user input at the user device.