A network access control list (ACL) is a set of rules that controls the inbound and outbound traffic for a network interface or a subnet. A deny rule can be used to block or filter the traffic from a specific source or destination, such as an IP address, a port number, or a protocol. By adding a deny rule to the network ACL, a systems administrator can prevent the communication between the compromised instance and the attacker, or between the compromised instance and other instances or servers. This can help to contain the security incident and limit the potential damage or data loss. A deny rule can also be used to isolate the compromised instance for further investigation or remediation. Reference: CompTIA Cloud+ CV0-003 Study Guide, Chapter 5: Maintaining a Cloud Environment, page 222-223; What is a network access control list (ACL)?.