Elastic Compute Service:Secondary private IP

Secondary private IP

If the single primary private IP of an elastic network interface (including the primary network interface and secondary elastic network interfaces) on an ECS instance is not sufficient for your requirements, you can assign multiple secondary private IPs to it. The figure below illustrates the relationship between the network interfaces and private IPs of an ECS instance.

Use cases

Common use cases for secondary private IP addresses include:

• Multi-application hosting: Host multiple services on a single ECS instance by assigning a separate secondary private IP address to each application, improving instance utilization.

• Failover: When an instance fails, you can detach the secondary ENI from the failed instance and attach it to a standby instance. This redirects traffic for the associated secondary private IP addresses to the standby instance.

• Load balancing: Assign multiple secondary private IP addresses to an instance for load balancing, improving system scalability and performance.

Notes

• The maximum number of private IP addresses that you can assign to an elastic network interface depends on its status.

  • If an elastic network interface is Available, you can assign up to 10 private IP addresses to it.

  • If an elastic network interface is Bound, the instance type determines the maximum number of private IP addresses you can assign. For details, see the Private IPv4 Addresses per ENI column in the Instance Specification Details section of the instance family documentation.

• VPC security groups support a limited number of private IP addresses. Therefore, plan your secondary private IP address assignments accordingly. For more information, see Security Group Limits.

Step 1: Assign a secondary private IP address

On the Manage ENI IP Addresses page, you can add or delete secondary private IP addresses for a target elastic network interface, which can be the primary network interface or a secondary elastic network interface of an ECS instance.

1. Go to ECS Console - Elastic Network Interfaces.

2. In the upper-left corner of the page, select a region and resource group.

3. On the ENIs page, find the elastic network interface to manage, and then click Manage ENI IP Addresses in the Actions column.

4. In the Manage ENI IP Addresses dialog box, click Add in the Secondary Private IPv4 Address or IPv6 Supported section, and then select a method to assign an IP address.

   • Auto-assign: The system automatically assigns an available IP address from the private IPv4 or IPv6 CIDR block of the elastic network interface.

   • Manually Enter: Enter a secondary private IP address. The address must be within the private IPv4 or IPv6 CIDR block of the elastic network interface.

5. Click OK.

6. Verify that the secondary private IP address is correctly assigned to the target elastic network interface.

   You can confirm this on the ECS instance's elastic network interface details page.

   The ENI is bound to one primary private IP address and two secondary private IPv4 addresses.

Step 3: Configure the secondary private IP address

After assigning a secondary private IP address to an instance, you must also configure it in the instance's operating system. Configuration steps vary by operating system and IP address type.

sudo vi /etc/sysconfig/network/ifcfg-eth0

# Disable DHCP and use static configuration
BOOTPROTO=static
STARTMODE=auto
USERCONTROL=no

# Add a numeric suffix for multiple IP addresses
IPADDR_1=192.168.1.201
NETMASK_1=255.255.255.0
LABEL_1=

IPADDR_2=192.168.1.202
NETMASK_2=255.255.255.0
LABEL_2=0

IPADDR_3=192.168.1.203
NETMASK_3=255.255.255.0
LABEL_3=1

cat /etc/sysconfig/network/routes

default 192.168.1.253 - -

Unassign secondary private IP addresses

This topic describes how to unassign one or more secondary private IP addresses from an elastic network interface (ENI) when they are no longer needed.

Automatic network configuration with cloud-init

cloud-init is an open-source tool that initializes a Linux operating system by setting passwords, configuring hostnames, and running custom scripts. cloud-init is installed by default on all Alibaba Cloud public images. When an ECS instance starts, cloud-init automatically generates the system network configuration.

• Notes about cloud-init in network configuration files

  A note in the network configuration file indicates that cloud-init automatically generated the file. The following example shows the network configuration file /etc/sysconfig/network-scripts/ifcfg-eth0 for Alibaba Cloud Linux 3.2:

  # Created by cloud-init on instance boot automatically, do not edit.
  
  BOOTPROTO=dhcp
  DEVICE=eth0
  HWADDR=00:16:3e:09:7d:73
  ONBOOT=yes
  TYPE=Ethernet
  USERCTL=no

• Disable the network configuration feature of cloud-init

  • When to disable the network configuration feature of cloud-init: Disable the network configuration feature of cloud-init in the following scenarios. Otherwise, cloud-init may overwrite your custom network configurations when the system restarts.

    • Manually configuring a static network in the network configuration file created by cloud-init.

    • Managing the network with another network management service, such as NetworkManager.

  • How to disable the network configuration feature of cloud-init: Create the /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg file and add the configuration network: {config: disabled}.

    Important

    For more information about cloud-init, view the /etc/cloud/cloud.cfg.d/README file on your Linux instance.

IP address configuration

You can configure an IP address using two methods: DHCP and static configuration. Choose a method when you configure a secondary private IP address.

• Static configuration: You manually configure network parameters, such as the subnet mask, default gateway, and DNS server address.

• DHCP dynamic assignment: DHCP is a network protocol that automatically provides devices on a network with configuration information, such as an IP address, subnet mask, default gateway, and DNS server address. You can also configure a secondary private IP address for a Linux instance configured to use DHCP.