i2c address of slave chip
The slaves are hard-wired. Most of the i2c chips like eeproms, etc., allow you to modify one or two bits of the address, depending I believe more on the pins available on the chip package rather than for any other reason. So, if you're using a chip with built-in i2c, look at its specification and it will tell you the possible addresses it can have. If you're doing a software i2c slave, choose an address yourself, respecting that there can only be one slave at each address, and respecting the limitations on address of any other chips on the bus.