Mobile Device Identifiers, or Mobile Device IDs are a unique identifier which can be used to identify a mobile device. They can usually only be accessed via an app and not from the mobile web. Advertisers use these identifiers to determine if they have already served an ad to a specific user and retarget or frequence cap the ads shown to that user. Ultimately, passing device id back to advertisers will help mobile publishers improve the mobile app monetization.

Mobile Device Identifiers have changed over time. Previously Android ID and UDID were used as device IDs, but due to privacy concerns Apple, Android and Windows have all moved to a format that gives users more control over their privacy through the use of a new Advertising ID.

Apple stopped accepting new apps, to the Apple Store on May 1, 2013, that accessed the iOS device’s UDID (Universal Device ID) due to security and privacy concerns. In its place they introduced a new advertising ID – IDFA.

In October of 2013 Google announced they would be replacing the Android ID with a new Android Advertising ID (aka Google Advertising ID) to offer increased security and privacy options for users.

Microsoft also announced the addition of a new Advertising ID in October of 2013 for Windows Phone devices.

There are lots of different ID acronyms you may hear: Android ID, Android Advertising ID, Google Advertising ID, UDID, IDFV, IDFA, etc. The list is long and can be confusing for a publisher trying to figure out what device ID information they need to send for an ad request. Below is a short description of what these IDs are and which ones should be used.

Android ID – this was the old unique identifier available on Android devices prior to the KitKat OS release. Mobile devices running KitKat or an earlier OS release will only have access to this device ID. Typically advertisers want the newer Android Advertising ID, but if the only device ID available is the Android ID then they may accept that instead. Check with your advertiser to see if they want the Android ID if there is no Android Advertising ID available.

Android Advertising ID (aka Google Advertising ID) – this is the current unique identifier used for Android devices. Most advertisers want this ID and you should be using it. One exception are Android apps available through the Amazon store. These apps do not have the Google Play Services SDK installed, so there is no Android/Google Advertising ID on these devices even though they may be running KitKat or a later OS version. These apps account for a small portion of the mobile traffic so you may not need to worry about them, but if they start driving a larger portion of your traffic you may want to speak with your advertiser to discuss what information to provide. Amazon may implement their own Advertising ID in the future or the release of their new Fire OS may resolve this issue.

UDID (aka Universal Device ID) – this was the old unique identifier for iOS. It is no longer used and has been replaced with the iOS IDFA ID. Apple no longer accepts apps in their store that access this identifier.

IDFA (aka identifierForAdvertising) – this is the current unique identifier for iOS and is what should be used to identify devices.

IDFV – (aka identifierForVendor) – this is a publisher specific identifier. The same identifier is used for the same user on the same app on all devices.  But the same user on a different app will have a different IDFV. Prior to Windows 8.1 the device ID on Windows Phones behaved similar to this ID. A unique ID was available for each app on the same device.

Windows Advertising ID – the new unique identifier used on Windows phones running Windows 8.1 and later. This is an Advertising ID similar to the Android Advertising ID and the iOS IDFA and should be used to identify devices.

It can be confusing trying to understand all of the different unique identifiers on the different platform devices, but if you work with your advertiser they can help you identify which device IDs they expect. Passing the correct device ID will only improve your chances of a greater fill rate and revenue.