The owner of the a.b.c.d
netblock is also the owner of c.b.a.in-addr.arpa.
The reverse DNS query looks up a PTR
record for d.c.b.a.in-addr.arpa.
internally. (Still not too many years ago, you had to perform this manipulation yourself; it wasn't built into dig
and friends.)
There are semi-obvious problems with this when the delegated netblock is smaller than a /24; then the upstream will have to handle rDNS somehow. It's not too uncommon that they will provide approximate or outright incorrect information in this scenario. (https://www.rfc-editor.org/rfc/rfc2317 proposes a solution, but it is nowhere near ubiquitous in practice.)
http://en.wikipedia.org/wiki/Reverse_DNS_lookup has a much more detailed treatment if you require details.