Network Load Balancer (NLB) distributes client requests to one or more backend servers specified in server groups. NLB checks the availability of backend servers by performing health checks. You must specify a server group when you add a listener to NLB. After you create a listener, the listener uses the protocol and port that you specify to check for connection requests and forward the requests to the associated server group.
Server group types
|Server group type||Backend server type||Description||References|
|Instance||Allows you to specify Elastic Compute Service (ECS) instances, elastic network interfaces (ENIs), and elastic container instances as backend servers.||The backend servers and the server group must belong to the same virtual private cloud (VPC). The backend servers are used to receive requests distributed by NLB.||Add backend servers by specifying ECS instances, ENIs, or elastic container instances|
|IP||Allows you to specify backend servers by IP address.|
You can add IP addresses from the CIDR block of the VPC to which the server group belongs, across regions, or across VPCs. You can also add IP addresses of on-premises servers. The IP addresses in a server group are used to receive requests distributed by NLB.
Note When you add backend servers by specifying IP addresses, you can specify only private IP addresses. You cannot specify public IP addresses.
|Add backend servers by specifying IP addresses|
Important After a backend server of an NLB instance is released or after the private IP address of a backend server is modified, NLB does not update the status of the backend server. Before you release or modify an NLB backend server, we recommend that you remove the backend server from the NLB server group. This ensures that your service is not affected.