The security perimeter is the defined boundary around a protected area — physical (fence, wall, airlock) or logical (firewall, DMZ). Stricter security rules apply inside the perimeter than outside. You should clearly document the perimeter in the security zoning concept and control all crossing points. Physically this means access-control systems at every transition; logically it means firewalls and intrusion-detection systems. ISO 27001 Annex A 7.1 requires the definition of physical security perimeters. Bear in mind that a perimeter alone does not provide sufficient protection — defence in depth complements the concept.