Final answer:
The mechanisms acting as firewalls for a VPC and EC2 instances on AWS are Security Groups and Network Access Control Lists (NACLs). Security Groups provide stateful filtering for individual instances while NACLs offer stateless firewall capabilities at the subnet level.
Step-by-step explanation:
In the context of Amazon Web Services (AWS), two mechanisms act as a firewall for both your Virtual Private Cloud (VPC) and the instances within it: Security Groups and Network Access Control Lists (NACLs).
Security Groups are associated with individual instances and provide stateful filtering of inbound and outbound traffic. This means that if a response is allowed to an outbound request, the inbound response is also automatically allowed. Security groups operate at the instance level and allow you to specify allowable traffic based on protocol, port, and source/destination IP range.
Network Access Control Lists (NACLs) are stateless and operate at the subnet level of a VPC, providing an additional layer of security that acts as a firewall for controlling traffic into and out of one or more subnets. You must define rules for both inbound and outbound traffic with NACLs.
For a web application hosted on EC2 instances in AWS, using both Security Groups and NACLs can enhance the security by providing different layers of traffic control.