[ale] IN-ADDR.ARPA for a subnet

[Andrew Lancaster]

I would like to register a domain name for a subnet of a class b network
with a netmask of 255.255.255.192 (subnet 168.29.123.0) would this line
in named.boot be appropriate for this situation:

primary    0.123.29.168.IN-ADDR.ARPA    named.rev
 
or would I need something else here?, such as 123.29.168.IN-ADDR.ARPA? 
also, are there any other special considerations for assigning such a
subnet a domain name and/or reverse domain?