I'm not sure if I can answer you question in full but here some information that may help.
NAC can be setup to help protect you LAN.
For example If you have a wireless access point and want to allow some laptops access to a LAN that connects to the Internet but wish to ensure that the laptop users are who they say they are and not some drive by hackers the nac can sit between the wired lan and the wirelss access point.
The NAC pops up some sort of log application that forces the user to authenticate with a server such as Active directory. Once autenthicated the NAC also checks what rights that user has such as the user can or can't access the file server.