def prev_sib()
			if (self.network.addr == 0)
				return nil
			end
			
			shift = 32 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) - 1) << shift
			return IPv4Net.new(IPv4.new(addr), self.netmask)
		end
def prev_sib()
			if (self.network.addr == 0)
				return nil
			end
			
			shift = 32 - self.netmask.prefix_len
			addr = ((self.network.addr>>shift) - 1) << shift
			return IPv4Net.new(IPv4.new(addr), self.netmask)
		end
prev_sib returns the network immediately preceding this one or nil if this network is 0.0.0.0.