The invisible domain which handles reverse resolve requests exclusively. The IP addresses within this special zone are represented backwards. For example, 192.168.20.1 is actually 188.8.131.52.in-addr.arpa. This is due to the hierarchical nature of DNS (i.e. the owner of "192.in-addr.arpa" also owns "184.108.40.206.in-addr.arpa", but not "in-addr.arpa" itself). Naturally, this isn't true with IP addresses.