How many IPs do you have? If it is a lot maybe a class c, ask your provider to delegate the IPs to you, you can then do the reverse DNS yourself. If not you need to ask your provider to reverse them for you.
You don't need to allocate an IP address for reverse DNS. Reverse DNS is simply a reverse name mapping for each of your existing IP addresses (i.e. you can have 1 reverse entry for each of your 4 IPs).
Reverse entries allow "nslookup <ip address>" to return the FQDN assigned to that IP.
Originally posted by applepanther SO means i can't do this by myself. I have to take help of host.??
No matter HOW many IP addresses you have for your server, you must have your DC set this up, unless you maintain control of your IP space, and you own the ip addresses. The only ones authorized to make said change are the individual ip owners, which means that it's got to be done by the DC.
All you really need to have setup with RDNS is one, that would be the base ip of your system (usually). Here's why:
No matter what application comes out of your system, it'll all be processed through the base ip of your system, unless you've specifically told the application (ie: exim) to bind to another ip address.
The only place that RDNS truly matters is your MTA. Since your MTA sends mail OUT as the base ip , the only IP address you need to have configured with RDNS is your base ip address.
Make sense? Hopefully If not, ask questions
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Just to make the point, though in general yes your host will do reverse DNS for you a smaller than /24 allocation can be delegated. In general its much easier to just have a host do it for you though. Do a google search for "classless subnet reverse delegation" and you will see what I mean.