Provide network connectivity for your Auto Scaling instances using Amazon VPC - Amazon EC2 Auto Scaling
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Provide network connectivity for your Auto Scaling instances using Amazon VPC

We are retiring EC2-Classic. We recommend that you migrate from EC2-Classic to a VPC. For more information, see the blog post EC2-Classic Networking is Retiring - Here's How to Prepare.

Amazon Virtual Private Cloud (Amazon VPC) enables you to define a virtual networking environment in a private, isolated section of the Amazon Web Services Cloud. You have complete control over your virtual networking environment.

Within a virtual private cloud (VPC), you can launch Amazon resources such as Auto Scaling groups. An Auto Scaling group in a VPC works essentially the same way as it does on EC2-Classic and supports the same set of features.

A subnet in Amazon VPC is a subdivision within an Availability Zone defined by a segment of the IP address range of the VPC. Using subnets, you can group your instances based on your security and operational needs. A subnet resides entirely within the Availability Zone it was created in. You launch Auto Scaling instances within the subnets.

To enable communication between the internet and the instances in your subnets, you must create an internet gateway and attach it to your VPC. An internet gateway enables your resources within the subnets to connect to the internet through the Amazon EC2 network edge. If a subnet's traffic is routed to an internet gateway, the subnet is known as a public subnet. If a subnet's traffic is not routed to an internet gateway, the subnet is known as a private subnet. Use a public subnet for resources that must be connected to the internet, and a private subnet for resources that need not be connected to the internet. For more information about giving internet access to instances in a VPC, see Accessing the internet in the Amazon VPC User Guide.

EC2-Classic

If you created your Amazon Web Services account before December 4, 2013, it may allow you to choose between Amazon VPC and EC2-Classic in certain Regions. If you have one of these older accounts, you might have Auto Scaling groups in EC2-Classic in some Regions instead of Amazon VPC.

For information about migrating from EC2-Classic to a VPC, see the blog post EC2-Classic Networking is Retiring - Here's How to Prepare. For information about the differences between instances in EC2-Classic and a VPC, see EC2-Classic in the Amazon EC2 User Guide for Linux Instances.

To determine whether any of the Amazon Web Services Regions you use are still using EC2-Classic, open the Amazon EC2 console. If Supported platforms indicates only VPC, as shown in the following example, your Amazon Web Services account in the current Amazon Web Services Region uses the VPC platform, and uses a default VPC. The name of the default VPC is shown below the supported platform.


                    EC2-VPC platform
Tip

Any Auto Scaling groups created in a Region that has a default VPC can launch instances into the default VPC or a nondefault VPC, but not in EC2-Classic.

Default VPC

If you created your Amazon Web Services account after December 4, 2013 or you are creating your Auto Scaling group in a new Amazon Web Services Region, we create a default VPC for you. Your default VPC comes with a default subnet in each Availability Zone. If you have a default VPC, your Auto Scaling group is created in the default VPC by default.

You can view your VPCs on the Your VPCs page of the Amazon VPC console.

For more information about the default VPC, see Default VPC and default subnets in the Amazon VPC User Guide.

Nondefault VPC

You can choose to create additional VPCs by going to the VPC Dashboard page in the Amazon Web Services Management Console and selecting Create VPC.

For more information, see the Amazon VPC User Guide.

Note

A VPC spans all Availability Zones in its Amazon Web Services Region. When you add subnets to your VPC, choose multiple Availability Zones to ensure that the applications hosted in those subnets are highly available. An Availability Zone is one or more discrete data centers with redundant power, networking, and connectivity in an Amazon Web Services Region. Availability Zones help you to make production applications highly available, fault tolerant, and scalable.

Considerations when choosing VPC subnets

Note the following considerations when choosing VPC subnets for your Auto Scaling group:

  • If you're attaching an Elastic Load Balancing load balancer to your Auto Scaling group, the instances can be launched into either public or private subnets. However, the load balancer can be created in public subnets only.

  • If you're accessing your Auto Scaling instances directly through SSH, the instances can be launched into public subnets only.

  • If you're accessing no-ingress Auto Scaling instances using Amazon Systems Manager Session Manager, the instances can be launched into either public or private subnets.

  • If you're using private subnets, you can allow the Auto Scaling instances to access the internet by using a public NAT gateway.

  • By default, the default subnets in a default VPC are public subnets.

IP addressing in a VPC

When you launch your Auto Scaling instances in a VPC, your instances are automatically assigned a private IP address from the CIDR range of the subnet in which the instance is launched. This enables your instances to communicate with other instances in the VPC.

You can configure a launch template or launch configuration to assign public IPv4 addresses to your instances. Assigning public IP addresses to your instances enables them to communicate with the internet or other Amazon services.

When you launch instances into a subnet that is configured to automatically assign IPv6 addresses, they receive both IPv4 and IPv6 addresses. Otherwise, they receive only IPv4 addresses. For more information, see IPv6 addresses in the Amazon EC2 User Guide for Linux Instances.

For information on specifying CIDR ranges for your VPC or subnet, see the Amazon VPC User Guide.

Amazon EC2 Auto Scaling can automatically assign additional private IP addresses on instance launch when you use a launch template that specifies additional network interfaces. Each network interface is assigned a single private IP address from the CIDR range of the subnet in which the instance is launched. In this case, the system can no longer auto-assign a public IPv4 address to the primary network interface. You will not be able to connect to your instances over a public IPv4 address unless you associate available Elastic IP addresses to the Auto Scaling instances.

Network interfaces in a VPC

Each instance in your VPC has a default network interface (the primary network interface). You cannot detach a primary network interface from an instance. You can create and attach an additional network interface to any instance in your VPC. The number of network interfaces you can attach varies by instance type.

When launching an instance using a launch template, you can specify additional network interfaces. However, launching an Auto Scaling instance with multiple network interfaces automatically creates each interface in the same subnet as the instance. This is because Amazon EC2 Auto Scaling ignores the subnets defined in the launch template in favor of what is specified in the Auto Scaling group. For more information, see Creating a launch template for an Auto Scaling group.

If you create or attach two or more network interfaces from the same subnet to an instance, you might encounter networking issues such as asymmetric routing, especially on instances using a variant of non-Amazon Linux. If you need this type of configuration, you must configure the secondary network interface within the OS. For an example, see How can I make my secondary network interface work in my Ubuntu EC2 instance? in the Amazon Knowledge Center.

Instance placement tenancy

By default, all instances in the VPC run as shared tenancy instances. Amazon EC2 Auto Scaling also supports Dedicated Instances and Dedicated Hosts. However, support for Dedicated Hosts is only available for Auto Scaling groups that use a launch template. For more information, see Configure instance tenancy with a launch configuration.

Amazon Outposts

Amazon Outposts extends an Amazon VPC from an Amazon Region to an Outpost with the VPC components that are accessible in the Region, including internet gateways, virtual private gateways, Amazon VPC Transit Gateways, and VPC endpoints. An Outpost is homed to an Availability Zone in the Region and is an extension of that Availability Zone that you can use for resiliency.

For more information, see the Amazon Outposts User Guide.

More resources for learning about VPCs

Use the following topics to learn more about VPCs and subnets.