constexpr network_v6() noexcept;
Postconditions: this->address().is_unspecified() == true and prefix_length() == 0.
constexpr network_v6(const address_v6& addr, int prefix_len);
Postconditions: this->address() == addr and prefix_length() == prefix_len.
Remarks: out_of_range if prefix_len < 0 or prefix_len > 128.