AWS Certified Advanced Networking Specialty Practice Exam

Port 22 is commonly used for SSH (Secure Shell) access, which is typically employed to securely connect to servers for management purposes. In the context of this question, allowing access on port 22 from a specific NAT instance indicates a controlled environment where only traffic routed through that NAT instance is permitted.

This approach is crucial for securing the environment since it limits SSH access to a designated entry point, preventing unnecessary exposure of the instances to the wider internet and reducing the attack surface. By configuring the firewall or security group rules to accept incoming traffic only from that specific NAT instance, you establish a security boundary where only authorized traffic is allowed to reach your servers through SSH.

Access from any IP address would significantly compromise security by exposing the SSH service to potentially malicious actors. Access from the ELB or the Auto Scaling group, while potentially useful in some scenarios, doesn't provide the same level of control over who can initiate SSH connections, especially if these elements do not pass through the NAT instance. Thus, the configuration allowing access on port 22 specifically from a designated NAT instance aligns with best practices for network security and management.