Network Access Control (NAC) is a method of ensuring that only known devices are allowed to connect to your network and that they meet your network’s requirements before they are granted access.
This is NAC at its most basic level. However, today’s NAC is usually also capable of dictating each user’s level of access and of managing users’ access once they’re on the network.

NAC has a special place in a network security plan because, unlike a firewall, which offers perimeter protection, it monitors the inside of your network. A firewall stops the hacker in Poland from getting to your network through the Internet. NAC stops the hacker inside your building or in the parking lot from getting to your network through an Ethernet port or a wireless access point.

Even though a NAC appliance may also monitor network activity, enforce policies, control resources, and document security, the core function of NAC is to authenticate trusted devices and control who can access your network. It’s important to keep in mind that NAC is only part of a security plan, not a complete security measure. It doesn’t take the place of a firewall and won’t protect against data leaving through e-mail, printouts, or USB flash drives.