More precisely, "in-addr.arpa" is the reverse DNS lookup domain. The .arpa domain is now officially the "Addresses and Routing Parameters Area" (a "retronym" created after the fact; the original meaning was a reference to the no-longer-existent ARPAnet, for the U.S. Department of Defense's "Advanced Research Projects Agency"). It contains various infrastructure functions of which in-addr.arpa is the most famous and widely used, but a few others also exist like ip6.arpa for the new IPv6 lookups, and another one for the ENUM service (that creates connections between phone numbers and Internet addresses). Recently, uri.arpa and urn.arpa were created in accordance with RFC 3401 through 3404, which defined a new mechanism to resolve arbitrary URI and URN schemes through DNS.