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I am trying to clean up my AWS configuration and I want to know if particular subnets are actually used/have any resources in them.
I'm aware you can filter a list of a particular resource type (e.g. EC2 instances) by subnet id, through the AWS web interface, but I am not yet aware of all of the different resource types that may be used - so I am concerned I may miss something.
I have tried inspecting the subnet via the AWS CLI, but I can't see anything that clearly differentiates subnets that are in use and those that are not:
aws ec2 describe-subnets
This question deals with enumerating all IP addresses within a particular subnet's CIDR block, but it doesn't reveal how to show only active IP addresses (which I could presumably use to find the attached AWS resources and confirm a subnet is indeed in use).
This seems like it would be a common task, but I can find no AWS documentation or SO posts on how to do this. Perhaps there is something flawed in my approach.
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You can use AWS CLI to list all ENIs associated with the VPC and prettify the output using the --query parameter to get a resource list with the desired fields (AZ, instance-id, etc.).
Public subnets have a default route to an Internet Gateway; private subnets do not. So, to determine if a given subnet is public or private, you need to describe the route table that is associated with that subnet. That will tell you the routes and you can test for a 0.0.
aws ec2 describe-network-interfaces --filters Name=subnet-id,Values=subnet-id-here | grep Description (replace subnet-id-here with the subnet id in mind)
The above command will give you the names of resources in that subnet.
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