The 24xx256 is an I2C EEPROM and an easy way to extend the memory of your mircoc-controller by 256kbit, which is 32kb (32768 bytes). The 24LC256 version can be bought from SparkFun and it is very easy to program.

It is good to read the complete 24xx256 Datasheet , but here is a summary:

Byte	7	6	5	4	3	2	1	0	Function
1st	1	0	1	0	A2	A1	A0	R/W
2nd	ignored	Addr<14:8>
3rd	Addr<7:0>

There is a great tutorial on the dangerousprototypes site.

HiZ> m 4 3 1
I2C (mod spd)=( 0 2 )
Ready
I2C> W
Power supplies ON
I2C> P
Pull-up resistors ON
I2C>

I2C> [ 0xa0 0 0 1 3 2 ]
I2C START BIT
WRITE: 0xA0 ACK
WRITE: 0x00 ACK
WRITE: 0x00 ACK
WRITE: 0x01 ACK
WRITE: 0x03 ACK
WRITE: 0x02 ACK
I2C STOP BIT
I2C>

I2C> [ 0xa0 0 0 ] [ 0xa1 r:3 ]
I2C START BIT
WRITE: 0xA0 ACK
WRITE: 0x00 ACK
WRITE: 0x00 ACK
I2C STOP BIT
I2C START BIT
WRITE: 0xA1 ACK
READ: 0x01  ACK 0x03  ACK 0x02
NACK
I2C STOP BIT
I2C>