An iACL, or infrastructure access control list, is a packet filter applied to network infrastructure such as switches, routers, and firewalls. It is used to permit only trusted sources and necessary services while blocking all other traffic to sensitive control-plane ports and management interfaces. In practice, an iACL helps make sure that even if a service is listening, it is not reachable from the wrong network segment.
iACLs matter because many high-impact attacks depend on simple reachability. If an attacker can reach a vulnerable management port or internal service, they may be able to trigger remote code execution, steal configuration data, or disrupt routing and switching. Defenders use iACLs to reduce exposure, limit east-west movement inside data centers, and add a compensating control when patching must wait. They are not a replacement for fixes, but they can turn an internet- or internal-facing flaw into an unreachable one.