“arpa” stands for Address and Routing Parameter Area. It is a top-level domain (TLD) used primarily for the management of network infrastructure. The .arpa domain is historically significant as it derives its name from the Advanced Research Projects Agency (ARPA), which developed the ARPANET, an early precursor to the internet.
The .arpa TLD includes namespaces like in-addr.arpa for IPv4 reverse DNS lookups and ip6.arpa for IPv6 reverse DNS lookups. These are used to facilitate the reverse mapping of IP addresses to domain names, which is essential for various network services, including anti-spam measures.