Besides requiring services provided by the system, some instances also need to access the Internet to obtain information or download software. You can bind an EIP to a virtual NIC (port) of an instance so that the instance can communicate with external networks. However, assigning a public IP address to each instance consumes already-limited IPv4 addresses, incurs additional costs, and may increase the attack surface for a virtual environment. Therefore, enabling multiple instances to share one public IP address is a preferable and more feasible method. This can be done using SNAT.

The SNAT function maps private IP addresses in VPCs to public IP addresses, thereby allowing instances in VPCs to access the Internet. After the SNAT function is available to you, all instances in a subnet can use the configured EIP to access the Internet.