In this chapter, we will take a closer look at what physical security entails in the context of an ICS. We will look at the ICS security bubble analogy that I like to use during security engagements, to help visualize and explain the methodology behind securing legacy devices and systems that cannot be defended by regular means because of device restrictions or uptime requirements.

In this chapter we will discuss the following topics:

• The ICS security bubble analogy 
• Segregation exercise 
• Physical ICS security