What is NAT?
From AWS documentation:
A Network Address Translation (NAT) gateway is a device that forwards traffic from private subnets to other networks.
There are two types of NAT gateways:
- Public: Instances in private subnets can connect to the internet but cannot receive unsolicited inbound connections from the internet.
- Private: Instances in private subnets can connect to other VPCs or your on-premises network.
Each private or public NAT gateway must have a private IPv4 address assigned to it. Each public NAT gateway must also have an elastic IP (EIP) address (which is static public address associated with your AWS account) associated with it. Choosing a private IPv4 address is optional. If you don't choose a private IPv4 address, one will be automatically assigned to your NAT gateway at random from the subnet that your NAT gateway is in. You can configure a custom private IPv4 address in Additional settings.
After you create the NAT gateway, you must update the route table that’s associated with the subnet you chose for the NAT gateway. If you create a public NAT gateway, you must add a route to the route table that directs traffic destined for the internet to the NAT gateway. If you create a private NAT gateway, you must add a route to the route table that directs traffic destined for another VPC or your on-premises network to the NAT gateway.
When to use NAT?
From AWS documentation:
The instances in the public subnet can send outbound traffic directly to the internet, whereas the instances in the private subnet can't. Instead, the instances in the private subnet can access the internet by using a network address translation (NAT) gateway that resides in the public subnet. The database servers can connect to the internet for software updates using the NAT gateway, but the internet cannot establish connections to the database servers.
Note that NAT is required if instances in private subnet need to send a request (initiate a new connection) to the host in Internet. If request has reached private instance (via Application Load Balancer for example), then NAT is not required. See: amazon web services - Can a EC2 in the private subnet sends traffic to the internet through ELB without using NAT gateway/instance? - Server Fault
How to create NAT?
Private NAT gateway traffic can't reach the internet.
When assigning private IPv4 addresses to a NAT gateway, choose how you want to assign them:
- Auto-assign: AWS automatically chooses a primary private IPv4 address and you choose if you want AWS to assign up to 7 secondary private IPv4 addresses to assign to the NAT gateway. AWS automatically chooses and assigns them for you at random from the subnet that your NAT gateway is in.
- Custom: Choose the primary private IPv4 address and up to 7 secondary private IPv4 addresses to assign to the NAT gateway.
You can assign up to 8 private IPv4 addresses to your private NAT gateway. The first IPv4 address that you assign will be the primary IPv4 address, and any additional addresses will be considered secondary IPv4 addresses. Choosing private IPv4 addresses is optional. If you don't choose a private IPv4 address, one will be automatically assigned to your NAT gateway. You can configure custom private IPv4 addresses in Additional settings.
Secondary IPv4 addresses are optional and should be assigned or allocated when your workloads that use a NAT gateway exceed 55,000 concurrent connections to a single destination (the same destination IP, destination port, and protocol). Secondary IPv4 addresses increase the number of available ports, and therefore they increase the limit on the number of concurrent connections that your workloads can establish using a NAT gateway.
You can use the NAT gateway CloudWatch metrics ErrorPortAllocation and PacketsDropCount to determine if your NAT gateway is generating port allocation errors or dropping packets. To resolve this issue, add secondary IPv4 addresses to your NAT gateway.You can assign up to 8 private IPv4 addresses to your private NAT gateway. The first IPv4 address that you assign will be the primary IPv4 address, and any additional addresses will be considered secondary IPv4 addresses. Choosing private IPv4 addresses is optional. If you don't choose a private IPv4 address, one will be automatically assigned to your NAT gateway. You can configure custom private IPv4 addresses in Additional settings.Secondary IPv4 addresses are optional and should be assigned or allocated when your workloads that use a NAT gateway exceed 55,000 concurrent connections to a single destination (the same destination IP, destination port, and protocol). Secondary IPv4 addresses increase the number of available ports, and therefore they increase the limit on the number of concurrent connections that your workloads can establish using a NAT gateway.