ESN - Electronic Serial Number is a unique 32-bits identification number embedded by manufacturers in wireless phones. In addition to being programmed into the phone's microchip, the ESN typically is found on a label under the cellular phone's battery.
The ESN is automatically transmitted to a base station when a call is made. The number is used to authenticate the validity of the phone to prevent fraud. ESNs are currently mainly used with CDMA phones.
The first 8 bits originally represented the manufacturer code, leaving 24 bits for the manufacturer to assign codes to mobile devices. This system allowed for up to 256 manufacturers and up to 16,777,215 unique codes for each manufacturer’s mobile devices.
To allow for more manufacturers, the manufacturer code was extended to 14 bits, leaving 18 bits for the individual mobile device codes. With this system, the manufacturer could only assign up to 262,144 codes.
In 2006, manufacturers started running out of unique ESNs, requiring a transition to the new MEID standard to replace ESN.
ESN number is located under the battery, below the battery, on the back or bottom of the device.