IP addresses are an important means for users to access ECS instances, and for ECS instances to provide external services. Currently, classic IP addresses are uniformly distributed by Alibaba Cloud. They are divided into public and private IP addresses.

Private IP addresses

An instance is allocated with a private network card and bound to a specific private IP address. Private IP addresses are required and cannot be modified.

If a private IP address is changed independently in an operating system, communication in the private network will be interrupted.

Communication traffic through private IP addresses between instances in the same region is free. Private IP addresses can be used in the following scenarios:

• Load balancing of the Server Load Balancer
• Intranet mutual access between ECS instances
• Intranet mutual access between an ECS instance and another cloud service (such as OSS and RDS)

Public IP addresses

Each instance is by default configured with a public network interface card. Unlike private IP addresses, public IP addresses are optional. If you select a public network bandwidth greater than 0 Mbps when purchasing an instance, a public IP address will be allocated during creation of the instance.

Regardless of your selected billing method, you must select a public network bandwidth limit. The bandwidth limit you select will determine the limit of the outgoing bandwidth for the public network card.

Public network traffic will be charged. Public IP addresses can be used in the following scenarios:

• Mutual access between an ECS instance and the Internet
• Mutual access between an ECS instance and another cloud service

Multicast and Broadcast

ECS does not support multicast or broadcast.