The perimeter is the security boundary between an organization’s internal network and the external network (typically the internet). Physically, the perimeter includes firewalls, proxies, VPN gateways, and DMZ systems. In traditional network architecture, the perimeter served as the primary line of defense. Cloud adoption, remote work, and mobile devices have dissolved the traditional perimeter, which is why zero-trust architectures have gained importance. Solid perimeter security remains an important component of a defense-in-depth strategy. ISO 27001 Annex A.8.20 and A.8.21 address network security and the protection of network services.