Every I2C device has an address. When the device in control (called the 'bus master') sends an I2C command, the first thing it sends is the address of the device it wants to talk to.
The MCP23017 has three pins (15, 16, and 17) which set the address for that device. You wire the pins high or low for 8 different addresses on the same bus.
The datasheet for the chip (http://www.adafruit.com/datasheets/mcp23017.pdf
) has more details.
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