Since December 4, 2013, AWS supports EC2-VPC only. But before that, it was possible to create EC2-Classic. EC2-Classic and EC2-VPC both are totally different in many ways. ClassicLink is the only way to make communication possible between them within the same region. Enabling the ClassicLink option allows a VPC to communicate with the EC2 instances launched in EC2-Classic. Without this option enabled, resources in the VPC need to use the public IP address of the EC2-Classic instance or tunneling for communication. If you have any resources in EC2-Classic that require direct communication with VPC resources, ClassicLink can help.
Enabling ClassicLink allows you to use VPC security groups on the EC2-Classic instances and, in turn, this enables communication between instances in EC2-Classic and VPC using private IPv4 addresses. It is available in all AWS accounts that support EC2-Classic instances. There is no additional charge for using ClassicLink; however, standard data transfer...