Every inode on disk has a unique inode number associated with it.

It is a requirement that inode numbers be persistent across unmounts and reboots, so once an inode is written to disk its inode number is fixed.

For performance reasons it must be possible to quickly find an inode on disk using its inode number.

XFS uses the physical location of the inode on disk to encode the inode number, which makes finding the inode on disk using the inode number a trivial task.