A bastion host is a special-purpose computer on a network specifically designed and configured to withstand attacks. The computer generally hosts a single application, for example a proxy server, and all other services are removed or limited to reduce the threat to the computer. It is hardened in this manner primarily due to its location and purpose, which is either on the outside of a firewall or in a demilitarized zone (DMZ) and usually involves access from untrusted networks or computers. In the context of Alibaba Cloud, a bastion host can be used to securely access and manage multiple ECS instances in a VPC through the Internet. By applying an EIP and binding it to the bastion host, the administrator can use SSH or RDP protocols to log on to the bastion host from the Internet, and then use the same protocols to access other ECS instances in the VPC through the private network. This way, the administrator can avoid exposing all the ECS instances to the Internet, which would increase the risk of attacks and incur higher costs. The bastion host can also be configured with security policies and monitoring tools to enhance the protection of the ECS instances in the VPC. References: Bastion Host, Access an ECS Instance by Using a Bastion Host