Definition
A security group acts as a virtual firewall for cloud resources, controlling inbound and outbound network traffic. It defines a set of rules that determine which network packets are allowed to reach or leave an instance, such as a virtual machine or database. By specifying protocols, ports, and source or destination IP addresses, security groups enforce network access policies. This granular control is essential for isolating resources, preventing unauthorized access, and maintaining the integrity of cloud environments. For instance, a security group might allow SSH traffic only from a specific management IP address while blocking all other inbound connections. Unlike network access control lists (ACLs) which operate at the subnet level, security groups are stateful and apply directly to individual instances. Security groups are fundamental concepts covered in cloud certifications like AWS Certified Solutions Architect Associate and CompTIA Security+.