TLD .arpa
The top-level domain .arpa was originally created at the time when the old ARPAnet was transitioning into the Internet, with use of the TCP/IP protocols and DNS to combine a multiplicity of networks into one global network. It was appended to the hostnames of ARPAnet nodes to give them their initial domain address (it was common in those days to informally use domain-style addresses to refer to sites on different networks; .uucp and .bitnet addresses sometimes appeared even though those domains never existed in the Internet root), to be changed later as the sites acquired more appropriate permanent addresses depending on the type of organization (e.g., .edu for educational institutions, .gov for government sites, .com for commercial entities). Once this transition was finished, it was expected that this domain would ultimately be removed as unnecessary.
However, one of the domains placed in this TLD was in-addr.arpa, used for reverse lookups of IP addresses, and this became a permanent part of the network's infrastructure, preventing the removal of its TLD. Some thought of moving it to .int was done, but this was decided against; too much software was already hardcoded to use the existing address. Ultimately, the geeks behind the scenes of making the Internet run decided to reposition .arpa as the "Address and Routing Parameter Area", intended specifically for infrastructure elements such as in-addr.arpa and others such as ip6.arpa that were added later. RFC 3172 documented this status.
New second-level domains in .arpa are rarely added, and getting one into the system usually requires proposing a new standard through the RFC process.
