In order to do the reverse translation easily, a domain was created
that uses hosts' addresses as part of a name that then points to the
data for that host. In this way, there is now an 'index' to hosts'
RRs based on their address. This address mapping domain is called
IN-ADDR.ARPA. Within that domain are subdomains for each network,
based on network number. Also, for consistency and natural
groupings, the 4 octets of a host number are reversed.
我
猜
是index
的意思引自RFC1033 Page 9