DNS HOWTO : A simple domain. : Why reverse lookups don't work. : You've got a classless subnet
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4.5.2. You've got a classless subnet

This is a somewhat advanced topic, but classless subnets are very common these days and you probably have one unless you're a medium sized company.

A classless subnet is what keeps the Internet going these days. Some years ago there was much ado about the shortage of ip numbers. The smart people in IETF (the Internet Engineering Task Force, they keep the Internet working) stuck their heads together and solved the problem. At a price. The price is that you'll get less than a ``C'' subnet and some things may break. Please see Ask Mr. DNS at http://www.acmebw.com/askmrdns/00007.htm for an good explanation of this and how to handle it.

Did you read it? I'm not going to explain it so please read it.

The first part of the problem is that your ISP must understand the technique described by Mr. DNS. Not all small ISPs have a working understanding of this. If so you might have to explain to them and be persistent. But be sure you understand it first ;-). They will then set up a nice reverse zone at their server which you can examine for correctness with nslookup.

The second and last part of the problem is that you must understand the technique. If you're unsure go back and read about it again. Then you can set up your own classless reverse zone as described by Mr. DNS.

There is another trap lurking here. Old resolvers will not be able to follow the CNAME trick in the resolving chain and will fail to reverse-resolve your machine. This can result in the service assigning it an incorrect access class, deny access or something along those lines. If you stumble into such a service the only solution (that I know of) is for your ISP to insert your PTR record directly into their trick classless zone file instead of the trick CNAME record.

Some ISPs will offer other ways to handle this, like Web based forms for you to input your reverse-mappings in or other automagical systems.


DNS HOWTO : A simple domain. : Why reverse lookups don't work. : You've got a classless subnet
Previous: The reverse zone isn't delegated.
Next: A real domain example