The current subdomains of .arpa are as follows:
- in-addr.arpa: Used for reverse lookups of IP addresses. Each IP address corresponds to a domain of the form 18.104.22.168.in-addr.arpa, where the numbers are those of the IP address but in reverse order because the hierarchies go left-to-right in IP addresses but right-to-left in domain names.
- ip6.arpa: The equivalent function to in-addr.arpa for IPv6; defined in RFC 3152. Used for reverse lookups in the next-generation Internet protocol.
- e164.arpa: Defined in RFC 3245 as the structure used in the ENUM system to map between telephone numbers and Internet addresses. In the future this will facilitate all sorts of interesting stuff such as routing of phone calls through the Internet and the accessing of Internet sites by telephone. A process is now in progress to delegate the various country codes in the phone system to managers of the equivalent DNS names.
You probably won't get anything useful if you type any of these addresses into a Web browser; they're intended to be used, invisibly to the end user, in various protocols that need to look up different sorts of addresses on the Internet.