We’ve noticed some confusion around which identifier to use after upgrading to the new AdMob front-end, so we’d like to take this opportunity to explain the terminology and the format of the identifier that the Google Mobile Ads SDK expects for serving AdMob ads (DFP and Ad Exchange identifier formats remain unchanged).

Previously, we referred to this identifier as “publisher ID” if you were serving AdMob only, which represented a string of 15 hexadecimal characters starting with the letter “a”, such as a141234567890ab. If you were using mediation, we referred to the identifier as “mediation ID”, which represented a string of 16 character hexadecimal characters such as e123456789012345.

In the new AdMob, this identifier is referred to as “ad unit ID” and has a format of ca-app-<publisherId>/<slotname>. The term publisherId now represents a unique account identifier with the format pub-xxxxxxxxxxxxxxxx (16 digits) and slotname identifies a unique ad space and has the format nnnnnnnnnn (10 digits).

If you configure the Google Mobile Ads SDK with an incorrectly formatted ad unit, the ad request will fail with the error message: "Cannot determine request type. Is your ad unit id correct?". Make sure your ad unit has the format ca-app-pub-xxxxxxxxxxxxxxxx/nnnnnnnnnn without any whitespace.

Note that passing just the publisher ID from your new AdMob account, for example pub-1234567890123456, will return the error message above. Instead, you should find the full ad unit ID you wish to use from your apps in the Monetize tab on apps.admob.com.

As always, we’re available on our forum to answer any technical questions you may have about Google Mobile Ads. You can also find us on Google+.