def nth_next_sib(nth)
			if (nth < 0)
				return nil
			end
			
			shift = 128 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) + nth) << shift
			if addr > NetAddr::F128
				return nil
			end
			return IPv6Net.new(IPv6.new(addr), self.netmask)
		end
def nth_next_sib(nth)
			if (nth < 0)
				return nil
			end
			
			shift = 128 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) + nth) << shift
			if addr > NetAddr::F128
				return nil
			end
			return IPv6Net.new(IPv6.new(addr), self.netmask)
		end
nth_next_sib returns the nth next sibling network or nil if address space exceeded.