It’s interesting that if someone came up to your door, they more likely gonna knock on the door before coming in, and you won’t be having a sign saying please come in and then identify yourself. It’s gonna be the other way around, you need to know who’s that person before you letting him in. Same concept goes for Networking. On the network you gonna take the same approach to know whom the individuals are before you let them on the network. So here comes the role of (Network Access Control) NAC.