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Previously, Apple had all but disabled tracking of iPhone users by advertisers when it stopped app developers from utilizing Apple mobile device data via UDID, the unique, permanent, non-deletable serial number that previously identified every Apple device.For the last few months, iPhone users have enjoyed an unusual environment in which advertisers have been largely unable to track and target them in any meaningful way.In iOS 6, however, tracking is most definitely back on, and it’s more effective than ever, multiple mobile advertising executives familiar with IFA tell us. Note that Apple doesn’t mention IFA in its iOS 6 launch page.