802.1X is an IEEE standard for port-based network access control. A device must authenticate against an authentication server (typically RADIUS) before being granted network access. Only after successful authentication does the switch or access point open the port.
In an ISMS context, 802.1X is a key measure for implementing ISO 27001 Annex A controls A.8.20 (Network Security) and A.8.1 (User Endpoint Devices). In enterprise networks, it prevents unknown devices from gaining access to internal resources. This is especially relevant in environments with many physical network ports — conference rooms, production floors, shared workspaces. A typical deployment involves three components: supplicant (client), authenticator (switch), and authentication server (RADIUS).