Web applications, wireless access,
network interconnectivities, and VPNs have made the perimeter a much more complicated concept than it was a
couple of years ago.
A layered approach requires implementing security solutions at different spectrums of the network. Another
similar concept is islands of security. To implement islands of security, do not restrict your thinking to perimeter
security. Do not depend on just one method for your security. You should, instead, have layers of
protection??”perimeter, distribution, core, and access layer. Figure 1-4 illustrates a basic multilayered security
mechanism, which is designed to protect the data flow in the system.
Figure 1-4. Layers of Defense
This layered approach is related to the technology of an environment and the complexity of each of the
technologies at each layer. The complexity comes from different protocols, applications, hardware, and security
mechanisms that work at one or more of the seven layers in the OSI model. Just as there are different levels
within an environment, different types of attacks can occur at each level and would require respective
countermeasures.
The Domino Effect
The OSI reference model was built to enable different layers to work independently of each other.
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