Infrastructure Domain Names .arpa
.arpa was the first top-level domain in the domain name system. The .Arpa domain extension was originally an acronym for the Advanced Research Projects Agency (ARPA), a faction of the United States Military. It was Intended for temporary use, to serve as a transition mechanism for particular hostnames in the Advanced Research Projects Agency Network, ARPANET. It was the considered to be the first network to implement the TCP/IP protocol suite which served as the basis for the internet.
Fast-forward to the internet as we know it and the .arpa TLD is used exclusively for technical purposes and now stands for Address and Routing Parameter Area. A set of hostnames were phased out as the internet evolved to be replaced by newly categorized TLDs including .edu, .com and .org. The sub-domain in-addr.arpa remains active fro reverse DNS queries of internet protocol addresses. For historical reasons, the .arpa domain is sometimes considered a generic top-level domain.