A perimeter divides an area into two. The act of drawing that line changes the character of each area.

In planning for security or emergency preparedness, that perimeter represents a barrier to risk. Each thin line on a site plan is a combination of physical barriers, policies, communications, and technologies that control access to that area.

With the increasing risks of network intrusions, security perimeters have become more complex. The security perimeter is no longer tied to the physical boundaries of the building but exists at any point where the corporate or security network is accessible.