How can the established keyword in an ACL entry be used?1 pointto permit only the returning TCP packets from an already existing TCP connection, and deny the initial TCP packet of a new session from an untrusted networkto permit both the initial TCP packet of a new session and the returning TCP packets from an existing TCP connectionto permit only the initial TCP packet of a new sessionto change a router into a true stateful firewall controlling the access on a session-by-session basis

The established keyword in an ACL entry can be used to permit only the returning TCP packets from an already existing TCP connection, and deny the initial TCP packet of a new session from an untrusted network. This is achieved by including the established keyword in the ACL rule that is applied to the incoming traffic.

To permit both the initial TCP packet of a new session and the returning TCP packets from an existing TCP connection, the established keyword should not be used in the ACL rule. This allows all TCP packets, regardless of their status, to pass through the ACL.

If the goal is to permit only the initial TCP packet of a new session, the established keyword should be excluded from the ACL rule. This will block any returning TCP packets from an existing TCP connection.

Using the established keyword in an ACL entry can also be used to change a router into a true stateful firewall controlling the access on a session-by-session basis. By including the established keyword in the ACL rule, the router will keep track of the state of each TCP connection and only allow returning packets from established connections to pass through. This enhances the security of the network by ensuring that only legitimate traffic is allowed.