Inode has 13 block addresses.
The first 10 are direct block addresses of the first 10 data blocks in the file.
The 11th address points to a one-level index block.
The 12th address points to a two-level (double in-direction) index block.
The 13th address points to a three-level(triple in-direction)index block.
This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.