How tiff image IFDs are stored in memory

Can someone explain how IFD(Image file directory) is stored in memory when accessing tiff image on the computer?

I am using tiff 4.0.8 (libtiff) in Ubuntu 14.04.
I have an image with 17 tag entries in 1 IFD.

Is there any code which handles the storing memory of IFDs in the libtiff library (tiff 4.0.8)?