Network Access Control (NAC) is a security solution that is designed to control and monitor access to a network. It is used to ensure that only authorized users and devices are allowed access to the network, and to prevent unauthorized access or attacks. 

NAC solutions typically involve the use of authentication and authorization protocols, such as RADIUS and TACACS, to verify the identity of users and devices, and to ensure that they are authorized to access the network. They may also involve the use of network access control lists (ACLs) and other security measures to control access to specific network resources or services. 

NAC solutions can be deployed as hardware appliances, software solutions, or cloud-based services, and can be used to protect networks of all sizes, from small business networks to large enterprise networks. They are an important part of a comprehensive security strategy, as they can help to prevent unauthorized access to the network and protect against attacks.