A security group is for use with instances either in the EC2-Classic
platform or in a specific VPC. For more information, see Amazon EC2 Security Groups
in the Amazon Elastic Compute Cloud User Guide and Security Groups for Your VPC
in the Amazon Virtual Private Cloud User Guide.
EC2-Classic: You can have up to 500 security groups.
EC2-VPC: You can create up to 500 security groups per VPC.
When you create a security group, you specify a friendly name of your
choice. You can have a security group for use in EC2-Classic with the
same name as a security group for use in a VPC. However, you can't have
two security groups for use in EC2-Classic with the same name or two
security groups for use in a VPC with the same name.
You have a default security group for use in EC2-Classic and a default
security group for use in your VPC. If you don't specify a security
group when you launch an instance, the instance is launched into the
appropriate default security group. A default security group includes a
default rule that grants instances unrestricted network access to each
other.
You can add or remove rules from your security groups using
AuthorizeSecurityGroupIngress, AuthorizeSecurityGroupEgress,
RevokeSecurityGroupIngress, and RevokeSecurityGroupEgress.