Network Access Control (NAC) is a policy system that decides which devices can join a network and what resources they may reach after connecting. It can check identity, device posture, certificate status, or whether a device meets security rules before granting access. Common controls include VLAN assignment, quarantine networks, and per-port authentication such as 802.1X.
NAC matters because network access is a trust boundary. In defense, it can block rogue laptops, isolate unmanaged IoT devices, and limit lateral movement from compromised endpoints. In attacks, criminals look for weak NAC setups, open switch ports, or overly broad exceptions so they can plug in an unauthorized device or move between segments. Effective NAC does not replace endpoint security, but it reduces the damage a bad device can do by enforcing least privilege at the point of entry.