[ excerpt from the man page ]
The rev-server directive provides a syntactic sugar to make specifying
address-to-name queries easier. For example
--rev-server=1.2.3.0/24,192.168.0.1 is exactly equivalent to
--server=/3.2.1.in-addr.arpa/192.168.0.1

It is not mentioned in the man page but the only prefixes that the
directive properly handles when dealing with IPv4 are /8, /16 and /24.
Specifying anything else as the same effect as specifying /16.

It is not a big deal for subnets on non-octet boundaries since they
cannot be represented using a single in-addr.arpa address. However, it
is unconvenient for /32 prefix while the analogous server directive
behaves as expected. E.g. the following server directive work
as expected:

    server=/42.10.168.192.in-addr.arpa/1.2.3.4

but the following does not:

    rev-server=192.168.10.42/32,1.2.3.4

and, in practice, the later behaves the same as:

    server=/168.192.in-addr.arpa/1.2.3.4

This strange behaviour is fixed by accepting /32 CIDR prefixes as a
valid value. Any other value will still be considered the same as /16.