Mobile Country Codes (MCC) are used in wireless telephone networks (GSM, CDMA, UMTS, etc.) in order to identify the country which a mobile subscriber belongs to. In order to uniquely identify a mobile subscribers network the MCC is combined with a Mobile Network Code (MNC).
https://android.stackexchange.com/questions/162448/difference-between-android-id-and-device-id
- DeviceID: This is the serial of the device, which should persist even a factory reset.
- AndroidID: This will be set at the first boot (either with a brand new device, or after a factory reset). As implicated, it does not survive a factory-reset
- There's a second "Android_ID" which is generated and used by the Google Services Framework (GSF), and thus often referenced as "GSF ID". Behaves basically like the AndroidID mentioned before (e.g. doesn't survive a factory-reset), and co-exists with it.
- Furthermore, there's the GAID (Google Advertising ID), which can be reset by the user via the Google Settings app.
During "normal operation" (i.e. as long as you not factory-reset your device or reset the GAID), all these IDs can be used to identify the device. When using multiple users (via user profiles) on a device, all except the DeviceID would even identify the user (profile).
To my knowledge, all those IDs are available across all Android versions (at least across those which are currently relevant, i.e. GB/2.3 and up). The last two obviously only if GApps are installed – so on devices running a Custom ROM without GApps, they might not exist.
https://en.wikipedia.org/wiki/Google_Cloud_Messaging
https://stackoverflow.com/questions/25526042/what-is-gcm-registration-id
A Registration ID is an identifier assigned by GCM to a single instance of a single application installed on an Android device. The device is assigned this identifier when it registers to Google Cloud Messaging. The GCM documentation doesn't specify what information is encoded in this identifier.
In some older Android versions (pre-4.0.4), a Google account is required to register to GCM.
If multiple users use the same app on the same device, they will be have the same registration ID, since GCM doesn't care about which user logs-in to the app. It's up to your server to determine which user is currently logged in to your app in a certain device, and based on that knowledge to deliver only relevant GCM messages to that device.
