PTR Reverse Dns ? How to setup?

Using directadmin, how do i make /setup reverse DNS (PTR) entries for server main IP of a direct admin panel?

Setting up reverse DNS has been covered several times in these forums.

The first step is:

Did your upstream delegate the authority to you?

If not, then you can't do reverse DNS.

If so, then for a Class-C delegation, or a classless delegation?

If for a classless delegation, then:

a) what size

b) what's the name of the zone they delegated to?

Jeff