Where does Amazon publish the range of IP addresses used for EC2 instances by region?

Solution 1:

https://ip-ranges.amazonaws.com/ip-ranges.json

This question has been answered before, but here is the link to the forums, select the sticky link to the list of Ip ranges used by Amazon, it gets updated reliably when they add new information.

EDIT: Changed link, whenever the post is updated the link breaks, so just gave a static link to the forum page with the sticky, should be safer.

Solution 2:

To stay up to date you can now (2015-08-31) subscribe to AWS public IP address changes via SNS.

Subscribe to the topic:

arn:aws:sns:us-east-1:806199016981:AmazonIpSpaceChanged

Every time the addresses are changed, you get a message like this:

{
  "create-time":"yyyy-mm-ddThh:mm:ss+00:00",
  "synctoken":"0123456789",
  "md5":"6a45316e8bc9463c9e926d5d37836d33",
  "url":"https://ip-ranges.amazonaws.com/ip-ranges.json"
}

You can then download https://ip-ranges.amazonaws.com/ip-ranges.json and update your systems.

Solution 3:

A later list, published in 2012: https://forums.aws.amazon.com/ann.jspa?annID=1528