def prev_sib()
			if (self.network.addr == 0)
				return nil
			end
			
			shift = 128 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) - 1) << shift
			if addr < 0
				return nil
			end
			return IPv6Net.new(IPv6.new(addr), self.netmask)
		end
def prev_sib()
			if (self.network.addr == 0)
				return nil
			end
			
			shift = 128 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) - 1) << shift
			if addr < 0
				return nil
			end
			return IPv6Net.new(IPv6.new(addr), self.netmask)
		end
prev_sib returns the network immediately preceding this one or nil if this network is ::.