Newbie: How to do Reverse-DNS (with an 8-IP ISP-supplied subnet)?

Chris chris at
Tue Mar 28 20:04:09 UTC 2000

Hi all,

I've recently gotten DNS running on my network here. I'm basically a bind
and Unix (FreeBSD) newbie, but everything seems to be running reasonably
well. The one thing I've left out is Reverse DNS.

Reverse DNS (I'm not sure if that's the official name - should I call it
'reverse lookup'?) at first baffled me a little, but it now seems very
obvious. In general, any time you have a distributed database that's set up
to do name-value lookups, you can't really do a value-name lookups. You'd
need a second database to map values to names. (That's a sloppy use of
'name' and 'value' but hopfeully I'm being clear enough). So no problem -
the InterNIC maintains a sperate database that maps IP addresses to domain

Here's my problem - I have an ADSL account, with which I get 5 IP addreses.
Basically, I have a subnet of 8 IP addresses, with (I think) 3 being
reserved for the network, broadcast, and gateway IPs, leaving my 5. So how
do I set up reverse DNS for these IPs? Since the IPs belong to my ISP, my
ISP would currently be doing reverse DNS for the entire Class C that my
little subnet belongs to. That sounds to me like I have to somehow contact
my ISP, and say "could you please add these PTR records to your zone file?"
Is that really my only option? Also, while I have a decent understanding of
classless subnetting and its advantages, it seems that reverse-DNS is
completely, umm, non-classless, which seems pretty sub-optimal. Am I wrong
about this?

Finally - it's possible that my ISP will refuse to cooperate with my
reverse DNS needs. If they do, what are the ramifications? Is there a "What
happens if I can't do reverse-DNS" paper somewhere that will list the
problems I might face? (I couldn't find anything in the FAQ or by searching
the mailing list archives.)


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