Apple's UDIDs are basically a set of alphanumeric characters that are used to uniquely identify an iPhone or iPad. The numbers are designed to let application developers track how many users have downloaded their application and to gather other information for data analytics.
In 2010, The Wall Street Journal did an investigative report showing how application developers were using Apple's UDID to gather a lot of personal information about the device owner, including name, age, gender, device location and phone numbers. In response to concerns about the tracking, Apple earlier this year stopped allowing new iOS applications to track UDIDs.
With the new iOS 6 the company has gone one step further by eliminating UDIDs completely and replace with a set of three new devie identifiers. One is a vendor specific identifier that can be used by application vendors to recognize specific devices, another is designed for use by online advertisers and the third is an application-specific ID. Unlike UDID's, the new identifiers are not persistent and can be cleared, though the device has to be completely reset to get rid of the advertiser identities, Hall said.
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