Zettabyte

Because of irregularities in definition and usage of the kilobyte, the exact number could be any one of the following:

1. 1,180,591,620,717,411,303,424 bytes - 1024 times 1024 times 1024 times 1024 times 1024 times 1024 times 1024, or 2⁷⁰. This is 1024 times a exabyte. This is the definition used in computer science and computer programming
2. 1,000,000,000,000,000,000,000 bytes - or 10 ²¹.

A yottabyte is 1024 times a zettabyte.

To clarify the meaning(1) above, the International Electrotechnical Commission (IEC), a standards body, in 1998 defined new prefixes by combining the International System of Units (SI) prefixes with the word "binary." They did not provide a name for this number, although, it would probably be zebibyte. This naming convention has not, as of 2003, been widely adopted.

links: http://www.quinion.com/words/turnsofphrase/tp-kib1.htm

http://www.nist.gov/public_affairs/techbeat/tb9903.htm