An IP allowlist is an access control rule that permits connections only from preapproved source IP addresses or network ranges. Anything not on the list is blocked by default. In practice, it is used to restrict access to systems such as admin panels, VPN portals, APIs, and industrial control interfaces.
In cyber security, allowlisting reduces exposure by making a service reachable only from trusted networks, such as an operator’s office, a monitoring center, or a vendor gateway. This is especially important for OT environments, where direct internet access to PLCs or other controllers can create serious risk. However, an IP allowlist is not a complete defense: IP addresses can change, trusted hosts can be compromised, and remote users may connect through shared networks or NAT. For that reason, it works best alongside VPNs, strong authentication, segmentation, and logging.