Why does a AWS NAT Gateway require an ElasticIP?

Question

While creating NAT Gateway an Elastic IP Address is created in AWS. The AWS documentation (1) also mentions the same. Why is an Elastic IP Address required for NAT Gateway?

Answer 1

Simply stated, the EIP is required because that is the way the NAT Gateway feature was engineered.

It wouldn't make sense to have a dynamic address on a NAT device -- if the address changed, that would be guaranteed to break any sessions in progress... and the only way to allocate a static address is by allocating an Elastic IP address (EIP).

It's also common to have external vendors whitelist your servers (for access to their servers) by providing them with the EIP of your NAT Gateway, in which case a dynamic address from the public pool would be unacceptable.

There may also be internal, proprietary reasons related to the way EIPs function that made this requirement a necessity... but that is not documented, so such an assertion would be mere speculation. An example of this: the public pool addresses (like those auto-assigned to EC2 instances configured with a public IP address from the pool, not an EIP) might be engineered to the specific availability zone, or even a subset of a single availability zone (they do, after all, change, when an instance is stopped/started, implying that they might be dedicated to specific server bays within an AZ), while EIPs can migrate anywhere from one zone to another within a region. This strongly implies different internal topologies.

This requirement (constraint?), to me, seems insignificant: you shouldn't be charged for this EIP, and if you need to increase the maximum allowed number of EIPs in a region, you can submit a support request at no charge, describing your use case, to request a limit increase.