thanks to geoff for pointing me in the correct direction.

the rfc in question is rfc 2317. i'll outline what's required,
since it wasn't obvious to me.

here's an example setting up t2317 ptr records for and in the network

# first we have to have the correct ipnet setup.
# dns needs this information to find the netmask.

ipnet=coraidt1 ip= ipmask=/123
# other stuff you may need goes here.

# next we need to declare our dom. dns normally does this
# for us, but for 2317-style reverse lookups, we do it ourselves. soa=
refresh=3600 ttl=3600

# and the entries.

- erik