Amazon EC2 Cookbook

Configuring security groups

Security groups are like firewalls for your EC2 instances. If you don't specify the security group while creating instance in EC2-VPC, then AWS automatically assigns the default security group of the EC2-VPC to the instance. We can configure the inbound and outbound rules for security groups. We can also change these inbound and outbound rules while the instance is running. These changes are automatically applied.

For every VPC, we get a default security group, which we can't delete. You can't use a security group that you created for EC2-VPC when you launch an instance in EC2-Classic. You also can't use security group that you created for EC2-Classic, when you launch an instance in EC2-VPC. After you launch an instance ...