What is an IMEI?

The IMEI (International Mobile Equipment Identity) is a unique 17 or 15 digit code used to identify an individual mobile station to a GSM or UMTS network. The IMEI number provides an important function; it uniquely identifies a specific mobile phone being used on a mobile network. The IMEI is a useful tool to prevent a stolen handset from accessing a network and being used to place calls. Mobile phone owners who have their phones stolen can contact their mobile network provider and ask them disable a phone using its IMEI number. With an IMEI number, the phone can be blocked from the network quickly and easily.
It is important to note that swapping a SIM card will not stop a phone from being banned. IMEI numbers are stored in the phones themselves, not on the SIM cards.
An IMEI is only used to identify the device and does not relate to a specific individual or organization. Other numbers such as the ESN (Electronic Serial Numbers) and MEID (Mobile Equipment Identifiers) can link an individual to a phone. Usually, an IMSI number stored on a SIM card can identify the subscriber on a network.
There are several ways in which you can locate your mobile phone's IMEI number. The IMEI can usually be found on the handset, beneath the battery, sometimes printed on a small white label. Another way to easily locate your IMEI phone is to dial the following sequence of numbers into the handset: *#06#. If you have a Sony Ericsson mobile phone, you can retrieve the IMEI by pressing the following key sequence: right,*, left, left,*, left, *, left.

The Structure of an IMEI Number

IMEI numbers either come in a 17 digit or 15 digit sequences of numbers. The IMEI format currently utilized is AA-BBBBBB-CCCCCC-D:

• AA: These two digits are for the Reporting Body Identifier, indicating the GSMA approved group that allocated the TAC (Type Allocation Code).
  
• BBBBBB: The remainder of the TAC
  
• CCCCCC: Serial sequence of the Model
  
• D: Luhn check digit of the entire model or 0 (This is an algorithm that validates the ID number)
  

Central Equipment Identity Register (CEIR)

If a mobile handset is lost or stolen, the owner of the device can contact the CEIR (Central Equipment Identity Register) which will blacklist the device in all currently operating switches. This makes the lost or stolen handset unusable. The action of blacklisting stolen mobile handsets is seen as a deterrent to crime. It is possible, however, to change an IMEI with special tools and there are certain mobile networks that do not automatically blacklist handsets registered with the CEIR. Current statistics state that approximately ten percent of IMEI's in use today are not unique or have been reprogrammed (hacked).

CEIR Classification of Handsets

Color Meaning
White Valid Mobile Station
Grey Mobile Station to be tracked
Black Barred Mobile Station