The I2C bus selects a destination device by its address. In some cases, a device might have a fixed address, meaning only one device of that type can appear on the bus. However in most cases, a device's address is determined by its hardware configuration. In the image above, a 24LC256 serial EEPROM is depicted showing its three chip select pins which correspond to the three address bits in the I2C signal. The address the device will respond to depends on which pins are connected to the positive supply and which pins are connected to ground.
I²C Chip Select