DNS reverse mapping

Recently, I had to cope with configuring some reverse zones for subnets where the netmask is like 26 or 20 (IPv4). It's quite straightforward to do it with class C networks when it is sufficient to  reverse the order of network base of the address,  join the result with special domain in-addr.arpa and  create a reversed mapped zone file finally. If we had a network 192.168.1.0/24 then the reversed base of the network address would be 1.168.192 and the reversed zone 1.168.192.in-addr.arpa. 

The previous technique is well known for class C networks (B or A as well) when it is possible to split  it on its octet boundaries. If we have assigned a class C subnet which has less than 256 hosts we can't do it like this and we need to define the  network part and the host part of the address differently. The brute force way how to do it is to create a reversed zone for each host . The better way is to read through the RFC2317.

The RFC defines a classless allocation of subnets on non-octal boundaries with less than 256 hosts. Let's take a network 192.168.1.32/28 (subnet of network 192.168.1.0/24) where the network base is 192.168.1.32, the maximum number of host is 14 and the netmask is 255.255.255.240 (28 in CIDR notation). The next step is to reverse the network base which gives us 32.1.168.192 and join the result with the domain in-addr.arpa. It gives us a semi-reversed zone 32.1.168.192.in-addr.arpa. The final steps to construct the reversed zone are  not so clear. Take the first octet from the semi-reversed zone, substitute it with the netmask in CIDR notation and write it in the form first_octet/substituted_zone. In our example, we would get 32/28.1.168.192.in-addr.arpa (32 is the last octet and 28 is the netmask).

Why is the presented method useful? Even if the reverse zone creation is not so clear it helps to create only 1 zone file for 14 hosts in our example. If we had a network with netmask e.g. 25 we would be able to specify all 126 PTR records in one zone file. The method is obfuscated a bit but it eliminates  a creation of reversed zone file for each host.

It's important to realize the RFC2317 is for networks with 256 hosts or less  where  netmask  is from 24 to 32. For networks with more hosts there has to be used traditional delegation. If we have a network with netmask from 17 to 24 then we will have 1 zone file for each 256 hosts.  If we have a netmask from 9 to 16 then we can have 1 zone file per 65536 hosts.