VPC Network Address Translation

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VPC Network Address Translation When you associate an ENI with a public IP address, the ENI maintains its private IP address. Associating a public IP with an ENI doesn’t reconfigure the ENI with a new address. Instead, the Internet gateway maps the public IP address to the ENI’s private IP address using a process called network address translation (NAT). When an instance with a public IP connects to a host on the Internet, the host sees the traffic as originating from the instance’s public IP. For example, assume an instance with a private IP address of 172.31.7.10 is associated with the EIP 35.168.241.48. When the instance attempts to send a packet to the Internet host 198.51.100.11, it will send the following packet to the Internet gateway:

The Internet gateway will translate this packet to change the source IP address to the instance’s public IP address. The translated packet, which the Internet gateway forwards to the host, looks like this:

Likewise, when a host on the Internet sends a packet to the instance’s EIP, the Internet gateway will perform network address translation on the incoming packet. The packet that reaches the Internet gateway from the Internet host will look like this:

The Internet gateway will translate this packet, replacing the destination IP address with the instance’s private IP address, as follows:

Network address translation occurs automatically at the Internet gateway when an instance has a public IP address. You can’t change this behaviour. Network address translation as described here is also sometimes called one-to-one NAT because one private IP address gets mapped to one public IP address.

Also Read:- AWS Elastic Block Storage Volumes and It’s Features

Network Address Translation Devices

Although network address translation occurs at the Internet gateway, there are two other resources that can also perform NAT.
– NAT gateway
– NAT instance AWS calls these NAT devices.
The purpose of a NAT device is to allow an instance to access the Internet while preventing hosts on the Internet from reaching the instance directly. This is useful when an instance needs to go out to the Internet to fetch updates or to upload data but does not need to service requests from clients. When you use a VPC Network Address Translation device, the instance needing Internet access does not have a public IP address allocated to it. Incidentally, this makes it impossible for hosts on the Internet to reach it directly. Instead, only the NAT device is configured with a public IP. Additionally, the VPC Network Address Translation device has an interface in a public subnet.
Refer to Table 4.7 for an example.

When db1 sends a packet to a host on the Internet with the address 198.51.100.11, the packet must first go to the NAT device. The NAT device translates the packet as follows:


The NAT device then takes the translated packet and forwards it to the Internet gateway. The Internet gateway performs NAT translation on this packet as follows:


Multiple instances can use the same NAT device, thus sharing the same public IP address for outbound connections. The function that NAT devices perform is also called port address translation (PAT).

Related Products:- AWS Certified Solutions Architect | Associate

Configuring Route Tables to Use NAT Devices

Instances that use the NAT device must send Internet-bound traffic to it, while the NAT device must send Internet-bound traffic to an Internet gateway. Hence, the NAT device and the instances that use it must use different default routes. Furthermore, they must also use different route tables and hence must reside in separate subnets. Refer to Table 4.7 again. Notice that the instances reside in the Private subnet, and the NAT device is in the Public subnet. The default routes for these subnets would follow the pattern in Table 4.8.

Refer to the diagram in Figure 4.2 to see the relationship between both of the route tables. Recall that a route target must be a VPC resource such as instance, Internet gateway, or ENI. The specific target you choose depends on the type of NAT device you use: a NAT gateway or a NAT instance.


Read More :  https://www.info-savvy.com/vpc-network-address-translation/

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