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We are trying to expand our reliability setup using VPC in us-west-1. The 'a' and 'b' AZs work fine, but when I try to create subnets on 'c' it just fails with the message:

Value (us-west-1c) for parameter availabilityZone is invalid.
Subnets can currently only be created in the following availability zones:
  us-west-1b, us-west-1a.

Is this a transient failure or are there just only two VPC-enabled AZs in that region? If the latter, is there a list somewhere with how many AZs in each region are VPC-enabled?

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As Availability Zones grow over time, Amazon's ability to expand them can become constrained. If this happens, they might restrict you from launching an instance in a constrained Availability Zone unless you already have an instance in that Availability Zone. Therefore, your account might have a different number of available Availability Zones in a region than another account.

Use the describe-availability-zones command to describe your Availability Zones within the region.

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  • I got an error about a subnet being in a constrained zone when creating an ALB, but describe-availability-zones listed all zones including the subnet's zone.
    – qu1j0t3
    Commented Mar 2, 2017 at 17:24
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The letters are mostly meaningless.

Example:

Account 1 creates an instance in us-east-1a
Account 2 creates an instance in us-east-1b

Both instances are in the same availability zone, they just appear as different zones to the user.
It is just a way to represent that an instance is in a separate AZ from another.

Source: I was informed of this during AWS Systems Operations training in Sydney.

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    Regions and Availability Zones doc confirms this: "To ensure that resources are distributed across the Availability Zones for a region, we independently map Availability Zones to identifiers for each account. For example, your Availability Zone us-east-1a might not be the same location as us-east-1a for another account. Note that there's no way for you to coordinate Availability Zones between accounts"
    – Dusan Bajic
    Commented Feb 7, 2014 at 9:11
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    Yes, thats why I was careful to words it that "two of three". The psychological load balancing is irrelevant except for making the error message even harder to search for.
    – coderanger
    Commented Feb 7, 2014 at 9:18

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