Delegating /24's from a /19

2) Use DNAME, RFC 2672. Good luck.


3) Use RFC 2317. "I encourage my competitors to operate this way."

  Note: DNAME is equivalent to RFC 2317. In both cases this
  will break the customers expectation that they can just use for the PTR records.

  Note for reliable local reverse lookups when the external
  link is down they will need to slave the ISP's
  zone and well as manage the zone the CNAME / DNAME maps to.