How many tablets can bigtable store?

Question

From the paper of bigtable. bigtable I read this:

Each METADATA row stores approximately 1KB of data in memory. With a modest limit of 128 MB METADATA tablets, our three-level location scheme is sufficient to address 2^34 tablets.

I am confusing because 2^10 * 2^7 = 2^17

How could it store 2^34 tablets?

Answer 1

Figure 4: Tablet location hierarchy

The root tablet contains the location of all tablets in a special METADATA table.
Each METADATA tablet contains the location of a set of user tablets.

2^17 (METADATA tablet) * 2^17 (user tablets) = 2^34