Why do vpn/application gateways need to be in their subnet?

Azure requires VPN or application gateways to be in their own subnet because these gateways work as managed network appliances. The dedicated subnet keeps them isolated for security, management, and routing purposes. It allows Azure to control updates, avoid conflicts with user resources, and reserve IPs for scaling. This setup ensures stability and prevents interference with normal virtual machines in your network.

hi Longb1​ That’s an excellent observation — and it’s true that Azure’s approach is a bit different from what we’re used to in traditional on-prem networks.

In Azure Virtual Networks, resources like VPN Gateways, Application Gateways, and Azure Firewalls are managed services that act as network virtual appliances, rather than simple interfaces within your subnet.

Why they need their own subnet:

Isolation for managed routing and scaling

Azure gateways aren’t regular VMs — they’re platform-managed resources. Placing them in a dedicated subnet allows Azure to fully control routing, scaling, and IP allocation without conflicting with your own workloads.

Reserved IP ranges for internal operations

The gateway subnet must have a few unused IPs (usually at least a /27 range) so that Azure can deploy internal control plane components, health probes, and HA pairs. That’s why you can’t use all IPs in that subnet — Azure reserves several.

Separation of responsibilities

Gateways handle sensitive operations (encryption, routing, traffic inspection), so isolating them reduces the blast radius of configuration or security issues in your main subnets.

Predictable routing behavior

Having a distinct subnet ensures Azure’s routing tables can always unambiguously identify the gateway’s next hop. If it shared a subnet with workloads, route propagation and effective routes could conflict.

Why the subnet is small

Azure usually asks for a /27 or larger subnet (e.g., 10.0.255.0/27) because that provides enough IPs for HA, future scaling, and reserved system addresses — but it’s still small enough not to waste address space.

That’s a great explanation,Kidd.

To add a bit of background, in traditional networking the default gateway indeed lives within the same subnet as the hosts. Azure’s approach extends that logic as the gateway subnet acts as a dedicated control segment where system routes and reserved IPs can operate without overlapping with workload subnets.

https://learn.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-about-vpngateways
https://pingmynetwork.com/network/ccna-200-301/default-static-route

May related to:

1. Isolation for Managed Services

• Azure VPN and Application Gateways are managed services with their own control plane.
• By placing them in a dedicated subnet, Azure ensures clean separation between gateway infrastructure and user workloads.
• This isolation helps prevent conflicts with other resources and simplifies updates, scaling, and diagnostics.

2. Reserved IPs and Internal Routing

• Gateways often reserve multiple IP addresses in their subnet for internal components (e.g., health probes, load balancer front ends).
• Azure needs predictable space to allocate these resources without overlapping with your VMs or services.

3. Security and Policy Enforcement

• Dedicated subnets allow you to apply Network Security Groups (NSGs) and User Defined Routes (UDRs) specifically tailored to gateway traffic.
• This minimizes the risk of misconfigured rules affecting unrelated resources.

4. Operational Constraints

• Azure blocks deployment of other resources in gateway subnets to avoid interference.