Users are free to use these names as they would any other
reverse-mapping names. However, since there is no central
authority responsible for use of private addresses, users SHOULD
be aware that these names are likely to yield different results
on different networks.
Users are free to use localhost names as they would any other
domain names. Users may assume that IPv4 and IPv6 address
queries for localhost names will always resolve to the respective
IP loopback address.
Users are free to use “invalid” names as they would any other
domain names. Users MAY assume that queries for “invalid” names
will always return NXDOMAIN responses.