Defeating port knocking protection requires large-scale brute force attacks in order to discover even simple sequences. An anonymous brute force attack against a three-knock TCP sequence (e.g. port 1000, 2000, 3000) would require an attacker to test every three port combination in the 1–65535 range and then scan each port between attacks to uncover any changes in port access on the target system. Since port knocking is by definition stateful, the requested port would not open until the correct three-port number sequence had been received in the correct order and without receiving any other intervening packets from the source. The average case scenario requires approximately 141 trillion (655353 / 2) packets to determine a correct three-port number. This technique, in combination with knock attempt-limiting, longer or more complex sequences and cryptographic hashes, makes successful port access attempts extremely difficult.