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When an instance is created, a default Ethernet interface is attached to the system (eth0). You can create additional network interfaces and attach them to an instance.
If If an instance has only one Ethernet interface, a public IP address can be assigned to the interface automatically. However, this address will be removed by AWS as soon as the network is restarted after if a second interface has been is added to the instance and the instance is restarted. Be careful not to lose connectivity to your instance when changing the network configuration.
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After creating a network interface, you have to assign it to the instance where it will be used.
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Adding Restarting the instance after adding a second network interface will release any automatically assigned public IP address. Additionally, adding a second network interface to a non-Amazon Linux EC2 instance causes traffic flow issues. This occurs because both interfaces are in the same subnet, and when there is only one routing table containing one default gateway . Traffic that comes into and traffic that arrives at the second network interface leaves the instance using the first network interface or vice-versa. This is blocked by AWS because a mismatch between MAC address and IP address. To make the second interface work, perform the following steps:
Refer to the AWS documentation and to section Example of a More Complex Network Configuration. Failure to use the proper steps, may make your instance unreachable! |
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| Locate your network interface in the interface list and right-click on it. | The right-click opens the context menu. Select Attach. This will open the window to enter the necessary instance information. |  |
| Select your instance and confirm entry. | Select your instance from the drop-down list and click on Attach. The state of your interface will change from available to in-use. |  |
| Verify that instance has second interface. | Select your instance in the instance list. The description tab in instance details should now display two network interfaces:
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An Elastic IP address is a persistent, public IPv4 address to be used for one of your network interfaces or instances. You can associate an Elastic IP address with any instance or network interface in your account.
The advantage of associating the Elastic IP address with the network interface instead of directly with the instance is that you can move the network interface with its attributes easily from one instance to another.
The initial automatically assigned public IP address will be removed as soon as you restart the network or the instance after adding a network interface with an Elastic IP address to your instance. Do not restart your network or your instance before you are sure you can reach it via the Elastic IP address. The automatically assigned public IP address will also be disabled if you assign an Elastic IP address to the primary Ethernet interface of the instance.
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