Subnet
A subnet is a range of IP addresses in your VPC. You can create AWS resources, such as EC2 instances, in specific subnets.
Subnet Basics
Each subnet must reside entirely within one Availability Zone and cannot span zones. By launching AWS resources in separate Availability Zones, you can protect your applications from the failure of a single Availability Zone.
Subnet IP address range
When you create a subnet, you specify its IP addresses, depending on the configuration of the VPC:
- IPv4 only – The subnet has an IPv4 CIDR block but does not have an IPv6 CIDR block. Resources in an IPv4-only subnet must communicate over IPv4.
- Dual stack – The subnet has both an IPv4 CIDR block and an IPv6 CIDR block. The VPC must have both an IPv4 CIDR block and an IPv6 CIDR block. Resources in a dual-stack subnet can communicate over IPv4 and IPv6.
- IPv6 only – The subnet has an IPv6 CIDR block but does not have an IPv4 CIDR block. The VPC must have an IPv6 CIDR block. Resources in an IPv6-only subnet must communicate over IPv6.
Subnet types
- Public subnet – The subnet has a direct route to an internet gateway. Resources in a public subnet can access the public internet.
- Private subnet – The subnet does not have a direct route to an internet gateway. Resources in a private subnet require a NAT device to access the public internet.
- VPN-only subnet – The subnet has a route to a Site-to-Site VPN connection through a virtual private gateway. The subnet does not have a route to an internet gateway.
- Isolated subnet – The subnet has no routes to destinations outside its VPC. Resources in an isolated subnet can only access or be accessed by other resources in the same VPC.
Subnet routing
Each subnet must be associated with a route table, which specifies the allowed routes for outbound traffic leaving the subnet. Every subnet that you create is automatically associated with the main route table for the VPC.
Subnet settings
All subnets have a modifiable attribute that determines whether a network interface created in that subnet is assigned a public IPv4 address and, if applicable, an IPv6 address. This includes the primary network interface (eth0) that's created for an instance when you launch an instance in that subnet. Regardless of the subnet attribute, you can still override this setting for a specific instance during launch.
After you create a subnet, you can modify the following settings for the subnet:
- Auto-assign IP settings: Enables you to configure the auto-assign IP settings to automatically request a public IPv4 or IPv6 address for a new network interface in this subnet.
- Resource-based Name (RBN) settings: Enables you to specify the hostname type for EC2 instances in this subnet and configure how DNS A and AAAA record queries are handled.