Perimeter defenses are security measures designed to protect the outer boundary of a computer network from unauthorized access and cyber threats. Common examples include firewalls, intrusion detection systems, and network gateways. These tools monitor and control incoming and outgoing traffic, aiming to block malicious activity before it can reach internal systems. While perimeter defenses are a critical first line of defense, sophisticated attackers can sometimes bypass them, so they should be used alongside other security strategies for comprehensive protection.