July 2009 Archives

Summary: EC2 availability zone names in different accounts do not match to the same underlying physical infrastructure. This article explains a trick which can be used to figure out how to match availability zone names between different accounts.


As of the updating of this article (2011-12-23) Amazon EC2 (Elastic Compute Cloud) has twenty different availability zones in eight regions. A region can be thought of as a specific area of the world. An availability zone can be thought of roughly as a data center, defined such that no single failure scenario should affect two availability zones.

The current regions are:

  • us-east-1 - East coast of the United States (Northern Virginia)
  • us-west-1 - West coast of the United States (Northern California)
  • us-west-2 - West coast of the United States (Oregon)
  • sa-east-1 - East coast of South America (São Paulo, Brazil)
  • eu-west-1 - Western Europe (Dublin, Ireland)
  • ap-southeast-1 - SouthEast Asia (Singapore)
  • ap-northeast-1 - NorthEast Asia (Tokyo)
  • GovCloud - US Government use only

The availability zones in those regions are given the region name plus simple letters appended. For example:

  • us-east-1a
  • us-east-1b
  • us-east-1c
  • us-east-1d


  • eu-west-1a
  • eu-west-1b

When you start EC2 instances, you can specify an availability zone or let Amazon pick one for you. You always have a default region which can be overridden when starting instances.


In order to prevent an overloading of a single availability zone when everybody tries to run their instances in us-east-1a, Amazon has added a layer of indirection so that each account’s availability zones can map to different physical data center equivalents.

For example, zone us-east-1a in your account might be the same as zone us-east-1c in my account and us-east-1d in a third person’s account.

In fact, given the way that Amazon has set this up, I would not be surprised if Amazon may not occasionally reassign availability zone names which you are not currently using. For example, Amazon added the fourth availability zone in the us-east-1 region, but I suspect this might not be us-east-1d in all accounts (especially new ones).


On occasion, users sometimes want to know if instances in different accounts are running in the same availability zone. Or, users might want to know which availability zone is the one with which people are currently experiencing a particular problem.

You’ll often see users say that there is a problem in zone us-east-1a but this isn’t very helpful for other users because (as described above) that name only has significance within the original user’s account.

What would be helpful is a unique identifier which maps to the underlying physical infrastructure (e.g., data center) and can be mapped to the different availability zone names in each account.

I believe that Amazon may have inadvertently let slip a way to obtain this in the current implementation of the reserved instance offering ids. In my experiments so far, these seem to be tied to something outside of the account’s availability zones and, though the ids are the same, they are mapped to different availability zone names for different accounts.

To demonstrate this I’ve arbitrarily chosen the reserved instance offerings for m1.small, one year, Linux/UNIX.

To list the mappings for a single account, you can use a command like:

ec2-describe-regions | cut -f2 | while read region; do 
  ec2-describe-reserved-instances-offerings --region $region | 
    perl -ne 'print "$2 $1\n" if 
     m%\S+\t(\S+)\t(\S+)\tm1.small\t1y.\t.*\tLinux/UNIX(\t(.(?!Utilization)|Medium Utilization)*)$%'
done | sort


Here are the mappings for one of my accounts (let’s call it “Blue”):

ap-northeast-1a 4b2293b4-ed86-4e9c-a41d-b9aa6ab6d143 
ap-northeast-1b ceb6a579-a3ba-4488-ae42-59ca92a3d26f
ap-southeast-1a d586503b-ce55-4e1e-8012-f87395ccfb39
ap-southeast-1b 649fd0c8-66cb-4867-a124-70e7b38902b1
eu-west-1a c48ab04c-0bd0-4be9-8db5-a4bad61c6c58
eu-west-1b d586503b-7025-44e8-8487-09907b6b0e7e
eu-west-1c c48ab04c-ca5b-43be-999b-ed45d67a058e
sa-east-1a ceb6a579-ef87-4741-bbb5-f52911e182b4
sa-east-1b 248e7b75-96bb-4161-b50a-2f042094f022
us-east-1a 438012d3-80c7-42c6-9396-a209c58607f9
us-east-1b 60dcfab3-06bb-4b68-9503-53bf89823b5e
us-east-1c ceb6a579-757c-474b-b09b-52c84b605767
us-east-1d 649fd0c8-5d76-4881-a522-fe5224c10fcc
us-west-1a c48ab04c-446f-416d-b7e1-75db17b1354c
us-west-1b e5a2ff3b-79b4-4217-8c93-ebf1d633dd6e
us-west-1c 4b2293b4-def5-468b-ae97-63886afe7a09
us-west-2a 60dcfab3-d0b1-48f5-bb2a-031bba5ec5e5
us-west-2b ceb6a579-9c3b-4098-b745-c3e256d612c5

Here are the mappings for a different account (let’s call it “Red”):

ap-northeast-1a 4b2293b4-ed86-4e9c-a41d-b9aa6ab6d143
ap-northeast-1b ceb6a579-a3ba-4488-ae42-59ca92a3d26f
ap-southeast-1a 649fd0c8-66cb-4867-a124-70e7b38902b1
ap-southeast-1b d586503b-ce55-4e1e-8012-f87395ccfb39
eu-west-1a d586503b-7025-44e8-8487-09907b6b0e7e
eu-west-1b c48ab04c-0bd0-4be9-8db5-a4bad61c6c58
eu-west-1c c48ab04c-ca5b-43be-999b-ed45d67a058e
sa-east-1a 248e7b75-96bb-4161-b50a-2f042094f022
sa-east-1b ceb6a579-ef87-4741-bbb5-f52911e182b4
us-east-1a ceb6a579-757c-474b-b09b-52c84b605767
us-east-1b 438012d3-80c7-42c6-9396-a209c58607f9
us-east-1c 60dcfab3-06bb-4b68-9503-53bf89823b5e
us-east-1d 649fd0c8-5d76-4881-a522-fe5224c10fcc
us-west-1a e5a2ff3b-79b4-4217-8c93-ebf1d633dd6e
us-west-1b c48ab04c-446f-416d-b7e1-75db17b1354c
us-west-1c 4b2293b4-def5-468b-ae97-63886afe7a09
us-west-2a 60dcfab3-d0b1-48f5-bb2a-031bba5ec5e5
us-west-2b ceb6a579-9c3b-4098-b745-c3e256d612c5

From this, I theorize that availability zone us-east-1a in account Blue is the same as availability zone us-east-1b in account Red, but availability zones us-east-1d happen to be the same in both accounts.


Please note that this approach is not a documented feature of Amazon EC2. I may be misinterpreting what I am seeing and the mappings may be completely random for different accounts.

Amazon could at any time restructure how these values work so that the described offering ids cannot be used between accounts or do not map to any common infrastructure.

Use at your own risk and please post a comment if you find out any further data to support or disprove this theory.

[Update 2011-04-25: Tweaked command line to exclude more instance types Amazon has added. Updated info for current ids in current regions/zones.] [Update 2011-12-23: Tweaked command line to exclude more instance types Amazon has added. Updated info for current ids in current regions/zones.]

I will be speaking at the O’Reilly Open Source Convention (OSCON 2009) next week, giving a presentation on building custom images for Amazon EC2.

Though the presentation is primarily about doing it yourself with the practical portion focusing on Ubuntu images, I wanted to mention some of the third party services which can be used to build images of any kind for EC2.

If you run, use, or are aware of one of these AMI building services, please send me a message or post a comment on this entry.

And, for folks who still haven’t yet signed up for OSCON, here’s a code that gets you 30% off of registration: os09sphoc

Ubuntu AMIs

Ubuntu AMIs for EC2:

AWS Jobs

AWS Jobs