Network segmentation is the best security measure to reduce the risk of propagation when a cyberattack occurs, because it divides the network into smaller subnetworks that are isolated from each other and have different access controls and security policies. This limits the spread of malicious traffic and prevents attackers from accessing sensitive data or systems in other segments. A perimeter firewall, a data loss prevention (DLP) system, and a web application firewall are also useful security measures, but they do not prevent propagation within the network as effectively as network segmentation does. References: CISA Review Manual (Digital Version)1, Chapter 5, Section 5.2.3