Managing subnets in AWS makes me sad. Don’t get me wrong, AWS (as per normal) gives you full control over that kind of thing, I’m mostly complaining from an automation point of view.

Ideally, when you design a self contained environment, you want to ensure that it is isolated in as many ways as possible from other environments. Yes you can re-use shared infrastructure from a cost optimization point of view, but conceptually you really do want to make sure that Environment A can’t possibly affect anything in Environment B and vice versa.

As is fairly standard, all of our AWS CloudFormation templates use subnets.

In AWS, a subnet defines a set of available IP addresses (i.e. using CIDR notation, representing – Subnets also define an availability zone (for redundancy, i.e. ap-southeast-2a vs ap-southeast-2b), whether or not resources using the subnet automatically get an IP address and can be used to define routing rules to restrict access. Route tables and security groups are the main mechanisms by which you can lock down access to your machines, outside of the OS level, so its important to use them as much as you can. You should always assume that any one of your machines might be compromised and minimise possible communication channels accordingly.

Typically, in a CloudFormation template each resource will have a dependency on one or more subnets (more subnets for highly available resources, like auto scaling groups and RDS instances). The problem is, while it is possible to setup one or many subnets inside a CloudFormation template, there’s no real tools available to select an appropriate IP range for your new subnet/s from the available range in the VPC.

What we’ve had to do as a result of this, is setup a couple of known subnets with high capacity (mostly just blocks of 255 addresses) and then use those subnets statically in the templates. We’ve got a few subnets for publically accessible resources (usually just load balancers), a few for private web servers (typically only accessible from the load balancers) and a few for

This is less than ideal for various reasons (hard dependency on resources created outside of the template, can’t leverage route tables as cleanly, etc). What I would prefer, is the ability to query the AWS infrastructure for a block of IP addresses at the time the template is executed, and dynamically create subnets like that (setting up route tables as appropriate). To me this feels like a much better way of managing the network infrastructure in the cloud, keeping in line with my general philosophy of self contained environment setup.

Technically the template would probably have a dependency on a VPC, but you could search for that dynamically if you wanted to. Our accounts only have 1 VPC In them anyway.

The Dream

I can see the set of tools that I want to access in my head, they just don’t seem to exist.

The first thing needed would be a library of some sort, that allows you to supply a VPC (and its meta information) and a set of subnets (also with their meta information) and then can produce for you a new subnet of the desired capacity. For example, if I know that I only need a few IP addresses for the public facing load balancer in my environment, I would get the tool to generate 2 subnets, one in each availability zone in ap-southeast-2, of size 16 or something similarly tiny.

The second thing would be a visualization tool built on top of the library, that let you view your address space as a giant grid, zoomable, with important elements noted, like coloured subnets, resources currently using IP addresses and if you wanted to get really fancy, route tables and their effects on communication.

Now you may be thinking, you’re a programmer, why don’t you do it? The answer is, I’m considering it pretty hard, but while the situation does annoy me, it hasn’t annoyed me enough to spur me into action yet. I’m posting up the idea on the off chance someone who is more motivated than me grabs it and runs with it.


There is at least one downside that I can think of with using a library to create subnets of the appropriate size.

Its a similar issue to memory allocation and management. As it is likely that the way in which you need IP address ranges changes from template to template, the addressable space will eventually suffer from fragmentation. In memory management, this is solved by doing some sort of compacting or other de-fragmentation activity. For IP address ranges, I’m not sure how you could solve that issue. You could probably update the environment to use new subnets, re-allocated to minimise fragmentation, but I think its likely to be more trouble than its worth.


To summarise, I really would like a tool to help me visualize the VPC (and its subnets, security groups, etc) in my AWS account. I’d settle for something that just lets me visualize my subnets in the context of the total addressable space.

I might write it.

You might write it.

Someone should.