Setting up a secure and properly aligned fence requires much time and effort. Digging post holes can go smoothly if you don't encounter obstacles like rocks or roots. Big roots, rocky ground, or compacted dirt will complicate your work.
The perimeter of a 2D shape is its whole periphery. Fencing the entire field requires knowledge of its boundaries.
You can usually tell who owns a fence just by looking at the orientation of the frames; most builders and homeowners will orient the fence so that the "good" side faces the neighbouring property. The opposite neighbour should also do the same thing, creating a "good" and "bad" side to each house.
Third to half the total height of the post above ground should be the hole depth (i.e., a 6-foot tall fence would require a hole depth of at least 2 feet).