The in-addr.arpa notation is one way of representing the IP address, useful for the practical business of performing lookups. The other main notation type used for IPs in DNS is the Classless Internet Domain Routing or CIDR notation, used for specifying a range of IPs.

In CIDR notation the sections of the IP appear in the normal order, rather than the reversed order like in the in-addr.arpa notation, with a "network prefix" added that specifies how large the block is. This prefix identifies what proportion of the final section of the IP is used to point to the block, and how much is left for individual host addresses.

The details of how CIDR notation work are complex and only really of interest to professionals. For reverse DNS purposes you only need to understand that the larger the prefix number, the fewer individual addresses are within it. An IP block with a /24 added represents a block of 256 host addresses, while a /27 includes 32 addresses, and a /32 is one specific host address.

Your ISP will give you the IP range in either CIDR or in-addr.arpa notation, and you can use either to enter the range into our system for the purpose of reverse DNS.