in-addr.arpa showing on IP scan

Hello. Yesterday i went to a customers house to replace a small cheap repeater. Then the customer wanted to do a test, so he connected its laptop to the ethernet port and Started doing an ip scan with advanced IP scanner, but the list of IPs where showing like This: x.x.x.x.in-addr.arpa. why are they on reverse notation????? Never saw IP scanner displaying its findings on reverse notation. Can anyone enlighten me? Thanks