“A security group acts as a virtual firewall that controlls the traffic for one or more instances.” 1 The Amazon EC2 Security Groups are not just capable controlling traffic from an IP address, but also from all EC2 instances belong to a specific security group. I want to allow an instance belonging to one security group to access an instance belongs to another security group via a custom domain name (subdomain.example.com).

But when I configured the subdomain via Amazon Route 53, I have misconfigured it by assigning a A record, an IP address or the Elastic IP address of the instance. I should have used CNAME, and assigned the public DNS (the public hostname of the EC2 instance).