Virtual Networks and Subnets in AWS, Azure, and GCP

Now that we know what regions and availability zones are, let’s go back to Daniel Dib’s question:

As I understand it, subnets in Azure span availability zones. Do you see any drawback to this? Does subnet matter if your VMs are in different AZs?

Wait, what? A subnet is stretched across multiple failure domains? Didn’t Ivan claim that’s ridiculous?

TL&DR: What I claimed was that a single layer-2 network is a single failure domain. Things are a bit more complex in public clouds. Keep reading and you’ll find out why.

Each humongous public cloud provider has a different approach to virtual networks and subnets, and how they map into availability zones and regions. Let’s start with their differences.

Amazon Web Services

AWS Virtual Private Cloud (VRF if you wish) is limited to a single region. It can have many subnets, each subnet limited to a single availability zone. AWS is obviously enforcing very strict fault isolation.

VPCs and subnets in AWS

VPCs and subnets in AWS

Traffic between subnets within a region flows without limitations (apart from security rules), but of course AWS charges you for the privilege. To build a network across multiple regions you have to use VPC peering or a Transit Gateway.

AWS uses an interesting mix of L2- and L3-forwarding within a subnet, but it’s deterministic and unicast, so no harm done, and it’s all within a single failure domain anyway.


Like AWS VPCs, Azure Virtual Networks (VNet) are limited to a single region, but each subnet spans all availability zones.

VPCs and subnets in Azure

VPCs and subnets in Azure

It looks like the fault isolation just went out the window, but it’s not as bad as it looks. Layer-2 segments in Azure are extremely short – they exist only between a virtual machine and the adjacent hypervisor virtual switch. Each virtual switch is a full-blown IP router using host routes to reach other hosts within the same subnet.

Azure packet forwarding behind the scenes

Azure packet forwarding behind the scenes

Considering intra-subnet routing, it really makes no difference from the fault isolation perspective whether a subnet spans one or multiple availability zones, but it does have an impact on the ease of application deployment – more about that in the next installment.

With end-to-end IP routing, Azure subnets became a configuration convenience; all VMs connected to the same subnet share the same Network Security Group and the same User-Defined Route Table (in reality these two objects get inserted between a VM and the adjacent virtual switch).

Google Cloud Platform

GCP went one step further. VPCs are global resources (spanning multiple regions), while subnets are regional resources (spanning availability zones). IIRC from the last time I studied GCP while preparing for a customer workshop they also do pure IP routing within a VPC, but this is approximately as far as I could be bothered with GCP today (their documentation is great, but I have my reservations, and I’m not the only one).

GCP VPCs and subnets

GCP VPCs and subnets

More to Explore

If you need more details about AWS and Azure networking you’ll find them in these webinars:

Blog posts in this series

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