An Evil twin is a Wi-Fi access point (AP) of attack. It is a version of the rogue (phishing) attack. It attacks the sensitive information such as usernames, passwords and credit card details from other Wi-Fi users with the same SSID (Service Set Identifier) name. An attacker interrupts the wireless users by gathering personal or corporate information without user knowledge. Existing studies detect Evil Twin attack in 802.11 wireless networks. Our proposed system is to detect Evil Twin attack in NAC (Network Access Control), because Network Access Control (NAC) technology provides solution for both authen¬tication of users and devices much like 802.11 as well as validation of the security of devices that are connected to a network. The main purpose proposed study is to provide a preventive solution for Evil Twin Attack. Wi-Fi mask tool which is the extension of wire shark tool is used for prevention.