International Mobile Equipment Identity

The International Mobile Equipment Identity or IMEI is a number, usually unique, to identify GSM, WCDMA, and iDEN mobile phones, as well as some satellite phones. It is usually found printed inside the battery compartment of the phone. It can also be displayed on the screen of the phone by entering *#06# into the keypad on most phones.

The IMEI number is used by the GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network in that country. For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to "blacklist" the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone's SIM is changed.

The IMEI is only used for identifying the device and has no permanent or semi-permanent relation to the subscriber. Instead, the subscriber is identified by transmission of an IMSI number, which is stored on a SIM card that can (in theory) be transferred to any handset. However, many network and security features are enabled by knowing the current device being used by a subscriber.

The Structure of an IMEI Number/IMEISV (IMEI Software Version)

The IMEI (14 decimal digits plus a check digit) or IMEISV (16 digits) includes information on the origin, model, and serial number of the device. The model and origin comprise the initial 8-digit portion of the IMEI/SV, known as the Type Allocation Code (TAC). The remainder of the IMEI is manufacturer-defined, with a Luhn check (it is an algorithm used to validate ID) digit at the end. 

• 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 or EE: - Luhn check digit of the entire model or 0 (This is an algorithm that validates the ID number) / Software Version Number (SVN).

              

For example, the old style IMEI code 35-209900-176148-1 or IMEISV code 35-209900-176148-23 tells us the following:

TAC: 35-2099 - issued by the BABT (code 35) with the allocation number 2099

FAC: 00 - indicating the phone was made during the transition period when FACs were being removed.

SNR: 176148 - uniquely identifying a unit of this model

CD: 1 so it is a GSM Phase 2 or higher

SVN: 23 - The "software version number" identifying the revision of the software installed on the phone. 99 is reserved.

Reference

http://en.wikipedia.org/wiki/International_Mobile_Equipment_Identity