dualstack DNS name
dualstack DNS 在使用 EC2-Classic (Internet-facing, non-VPC) Elastic Load Balancer. 會回傳 IPv6 及 IPv4 的紀錄，若沒有使用 dualstack 的話，則會回傳 IPv4 的紀錄
name-1234567890.region.elb.amazonaws.com
name-123456789.region.elb.amazonaws.com
ipv6.name-123456789.region.elb.amazonaws.com
dualstack.name-123456789.region.elb.amazonaws.com
EC2-VPC
Load balancers in a VPC support IPv4 addresses only. [...]
EC2-Classic
Load balancers in EC2-Classic support both IPv4 and IPv6 addresses. [...]
The base public DNS name returns only IPv4 records. The public DNS name with the ipv6 prefix returns only IPv6 records. The public DNS name with the dualstack prefix returns both IPv4 and IPv6 records. We recommend that you enable IPv6 support by using the DNS name with the dualstack prefix to ensure that clients can access the load balancer using either IPv4 or IPv6.