AWS的安全组是一种虚拟防火墙，可以用于控制进入和离开AWS虚拟私有云(VPC)的流量。安全组是一种网络访问控制列表(NACL)的补充，因为安全组是在实例级别上进行管理的，而NACL是在子网级别上进行管理的。

AWS的安全组可以通过以下方式保护您的资源：

1. 控制入站和出站流量：您可以配置安全组规则来允许或拒绝入站和出站流量。入站流量是指流向实例的流量，而出站流量是指从实例流向其他网络的流量。
2. 根据协议和端口号进行过滤：您可以设置安全组规则来仅允许特定的协议和端口号的流量通过。例如，您可以配置安全组规则来仅允许SSH流量通过特定的端口。
3. 允许和拒绝IP地址：您可以设置安全组规则来仅允许特定IP地址的流量通过。例如，您可以配置安全组规则来仅允许来自内部网络的流量通过。
4. 允许和拒绝特定的安全组：您可以设置安全组规则来仅允许来自特定安全组的流量通过。例如，您可以配置安全组规则来仅允许来自特定应用程序的流量通过。
5. 随时更改规则：您可以随时更改安全组规则，以便在需要时立即更新您的安全策略。

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What is the purpose of a network ACL in AWS?

A. To control access to an EC2 instance based on the source IP address.
B. To control access to an S3 bucket based on the source IP address.
C. To control access to a VPC subnet based on the source IP address.
D. To control access to a Route 53 hosted zone based on the source IP address.

答案：C

解析：Network ACLs are used to control traffic to and from subnets in a VPC. They operate at the subnet level and evaluate traffic based on source and destination IP addresses, ports, and protocols. Network ACLs are stateless, meaning they do not track the state of a connection like a security group does. Instead, they evaluate each packet individually and can be used to block or allow traffic based on specific criteria.

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