Transmitting a Byte to a Slave Device
Once the start condition has been sent, a byte can be transmitted by the MASTER to the SLAVE.
This first byte after a start condition will identify the slave on the bus (address) and will select the mode of operation. The meaning of all following bytes depends on the slave.
A number of addresses have been reserved for special purposes. One of these addresses is reserved for the "Extended Addressing Mode". As the I2C bus gained popularity, it was soon discovered that the number of available addresses was too small. Therefore, one of the reserved addresses has been allocated to a new task to switch to 10-bit addressing mode. If a standard slave (not able to resolve extended addressing) receives this address, it won't do anything (since it's not its address).
If there are slaves on the bus that can operate in the extended 10-bit addressing mode, they will ALL respond to the ACK cycle issued by the master. The second byte that gets transmitted by the master will then be taken in and evaluated against their address.
Note: Even in 10-bit extended addressing mode, Bit 0 of the first byte after the Start condition determines the slave access mode ('1' = read / '0' = write).