The first thing that will need to be done to get reverse DNS working is to have the IP range delegated to our nameservers at the ISP. It is best to deal with this first as you will need to contact the ISP to find out exactly what the IP range you are working with, and can just have them do the delegation at the outset. So the first thing to do is contact your ISP to delegate the IP range as shown below for our example IP range.