Every byte on the SDA line must be eight bits long. The first byte after START contains seven bits of the slave device address and one bit that defines the direction of the transmission.
As any other data, the address is transmitted sequentially starting with the Most Significant Bit (MSB) and ending with the Least Significant Bit (LSB).
The direction bit has the following values:
0 – Write: the master transmits data to the slave;
1 – Read: the master receives data from the slave.