The in-addr.arpa and ip6.arpa domains handle the task of converting IP addresses back into names. For example, if you have the IP address 126.96.36.199 and want to see what name has that as its address, you would look up the PTR record for 188.8.131.52.in-addr.arpa. (note the reversal of the numbers).
For all networks within MNET system (e.g., 184.108.40.206/16, 220.127.116.11/16, and 2607.f830::/32), MN.IT Services’ DNS servers handle the reverse lookups. In order to guarantee correctness of these reverse entries, MN.IT Services constructs the reverse lookup information automatically from the forward information. For this method to work, MN.IT Services must be the primary server for the reverse lookup domains. Hence, MN.IT Services will not delegate any zones for in-addr.arpa or ip6.arpa domains.
As a result of this configuration, if there are DNS names outside of zones managed by MN.IT Services that are assigned IP addresses within the MNET system, MN.IT Services will add those outside names in for address to name conversion purposes (only).
Since any given in-addr.arpa or ip6.arpa address can3 only match to a single IP address, MN.IT Services will not normally configure more than one DNS name at any one IP address. If two or more DNS names must use the same IP address, all but one will be entered as CNAME records. See also section 9.