作为IPV6缺省路由对应的目的网络，2000::/3比::/0更精确，类似IPV4中0.0.0.0/1，128.0.0.0/1比0.0.0.0/0更精确。
The IPv6 Unicast space encompasses the entire IPv6 address range with the exception of FF00::/8. [RFC4291] IANA unicast address assignments are currently limited to the IPv6 unicast address range of 2000::/3.
Only one eighth of the total address space is currently allocated for use on the Internet, 2000::/3, in order to provide efficient route aggregation, thereby reducing the size of the Internet routing tables; the rest of the IPv6 address space is reserved for future use or for special purposes.
参考：
http://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.txt
https://forums.he.net/index.php?topic=1896.0
https://en.wikipedia.org/wiki/IPv6_address
