A perimeter appliance is a hardware or software device placed at the boundary of a computer network to protect it from external threats. Common examples include firewalls, intrusion prevention systems, and VPN gateways. These appliances monitor and control incoming and outgoing network traffic based on security rules, helping to block unauthorized access and prevent cyberattacks. By acting as the first line of defense, perimeter appliances are essential for safeguarding sensitive data and maintaining network security for organizations of all sizes.