It is the abbreviation of Internet Protocol Version 6. 
IPv6 is a new generation internet protocol succeeding the IPv4, which is widely used currently. 

The conventional standard protocol of fiber-optic line was IPv4 (PPPoE). However, due to increase of internet users requiring massive number of addresses, we are now migrating to IPv6 which has a vast address space. 

See the following page for IPv6 available with Asahi Net. 
IPv6 Connection Feature