### Designing Secure Networks Security designs in most environments rely on the concept of defence in depth. In other words, they are built around multiple controls design to ensure that a failure in a single control. With defence in depth in mind, it helps to understand that networks are also built in layers. The Open Systems Interconnection (OSI) model is used to conceptually describe how devices and software operate together through networks. Beyond the conceptual level, designers will create security zones using devices that separate networks at trust boundaries and will deploy protections appropriate to both the threats and security requirements of each security zone. ### Infrastructure Considerations The organisation's *attack surface*, or a device's attack surface, consists of the points at which an unauthorised user could gain access. This includes services, management interfaces, and any other means that attackers could obtain access to or disrupt the organization. Understanding an organisation's attack surface is a key part of security and infrastructure design. ### [[Network Design Concepts]]