See here triple indirect is given,
For triple indirect max possible file size is
= {(block size) /DBA}^3 * Block size
// DBA:Disk Block Address
So, 128 GB= (8 KB/DBA) ^3 *8KB
(DBA)^3 = 2^15 B
Cube root both side, and you will get
DBA =32 Bytes = 32*8 bits=256 bits