[Update: As predicted, these numbers are already out of date and Amazon has added more public IP address ranges for use by EC2 in various regions.]
Each standard Amazon EC2 instance has a public IP address. This is true for normal instances when they are first brought up and for instances which have had elastic IP addresses assigned to them. Your EC2 instance still has a public IP address even if you have configured the security group so that it cannot be contacted from the Internet, which happens to be the default setting for security groups.
Amazon has made public the EC2 IP address ranges that may be in use for each region.
From this information, we can calculate the absolute upper limit for the number of concurrently running standard EC2 instances that could possibly be supported in each region. At the time of this writing I calculate the hard upper limits to be:
|EC2 Region||Max Instances*|
An upper limit based on the IP address ranges does not tell you what the real number of possible instances is in a given EC2 region. It’s just an upper limit.
Amazon is sure to keep requesting, reserving, and announcing more IP addresses than is actively needed by EC2 at any point in time. Only they know the growth buffer percentage that they like to maintain.
Amazon may need to use different ranges of IP addresses for different groups of instances in different parts of their network, even in the same data center or availability zone, so they may publish and start using new ranges of IP addresses even before they are even near using up the capacity of previous ranges.
Amazon is free to add new IP address blocks to the list at any time as they keep growing, and they do. The specific numbers above could be out of date by the time you read this.
There are probably some IP addresses in each range that are reserved for various networking devices and protocols.
This calculation is for concurrently running EC2 instances. Many instances run for just a few minutes or hours and another instance, perhaps for another customer, can start up and use that same IP address moments later.
EC2 instances running inside Amazon VPC don’t necessarily use up an external IP address in Amazon’s EC2 public IP address ranges, so they are not included in the upper limits.
I am not a networking expert. I only play one at my startup.
Anything else I’m missing?
[Update 2012-12-27: Correct URL for Amazon’s latest IP address list.]