Final answer:
In 802.1X authentication, an access point or network switch acts as the authenticator that controls client access to the network by opening or blocking the port after authentication success or failure.
Step-by-step explanation:
When using 802.1X authentication, the device that controls physical access to the network, based on the authentication status of the client, is known as an access point (AP) in a wireless network or a network switch in a wired network. These devices operate as the authenticator and act as the intermediary between the client device seeking access and the authentication server, typically using a RADIUS server, that validates the credentials of the client. If the authentication is successful, the AP or switch opens the port to allow network access; otherwise, it blocks access.