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OpenStack Neutron Network Calculator

Plan Neutron tenant networks: usable addresses per subnet, port counts, floating IP demand and VXLAN segment usage.

Inputs

projects
networks
bits
instances
agents

Each DHCP agent consumes one port and one address per subnet.

routers
%

Neutron Ports Required

5,080ports

Usable Addresses per Subnet

251addresses

Tenant Networks

80networks

Floating IPs Required

1,680addresses

VXLAN Segment Usage

0.0005%

Subnet Sizing Verdict

Comfortable address headroom for growth

Step by step

  1. Values used

    Projects (tenants) = 40 projects; Networks per project = 2 networks; Subnet prefix length = 24 bits; Instances per network = 60 instances; DHCP agents per network = 2 agents; Routers per project = 1 routers; Instances needing a floating IP = 35 %

  2. OpenStack Neutron Network

    usable addresses = 2^(32 − prefix) − 2 − 1 gateway − DHCP agent ports; ports = instance ports + DHCP ports + router interfaces, and VXLAN usage = networks ÷ 16,777,216 available VNIs.

  3. Neutron Ports Required

    = 5,080 ports

  4. Usable Addresses per Subnet

    = 251 addresses

  5. Tenant Networks

    = 80 networks

  6. Floating IPs Required

    = 1,680 addresses

  7. VXLAN Segment Usage

    = 0.0005

  8. Subnet Sizing Verdict

    = Comfortable address headroom for growth

How it works

Neutron consumes more of a subnet than the instances do: the network and broadcast addresses go first, then the router gateway, then one address per DHCP agent — and redundant DHCP means two. Port count matters separately because ports, not addresses, are what the ML2 plugin and the database actually track. Port count drives Neutron server and agent load far more than instance count does, and a subnet sized exactly to the instance count will fail its first scale-out because the infrastructure ports were never budgeted.

Formula

OpenStack Neutron Network

usable addresses = 2^(32 − prefix) − 2 − 1 gateway − DHCP agent ports; ports = instance ports + DHCP ports + router interfaces, and VXLAN usage = networks ÷ 16,777,216 available VNIs.

DHCP agent ports
One address per agent per subnet, doubled for agent redundancy
router interfaces
One port per network plus the external gateway port
16,777,216
VNIs addressable by the 24-bit VXLAN segment ID

Frequently Asked Questions

How is OpenStack Neutron Network calculated?

usable addresses = 2^(32 − prefix) − 2 − 1 gateway − DHCP agent ports; ports = instance ports + DHCP ports + router interfaces, and VXLAN usage = networks ÷ 16,777,216 available VNIs. Neutron consumes more of a subnet than the instances do: the network and broadcast addresses go first, then the router gateway, then one address per DHCP agent — and redundant DHCP means two. Port count matters separately because ports, not addresses, are what the ML2 plugin and the database actually track.

Why does OpenStack Neutron Network matter?

Port count drives Neutron server and agent load far more than instance count does, and a subnet sized exactly to the instance count will fail its first scale-out because the infrastructure ports were never budgeted.

What values do I need to enter?

This calculator takes 7 inputs: Projects (tenants), Networks per project, Subnet prefix length, Instances per network, DHCP agents per network, Routers per project, Instances needing a floating IP. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Does VXLAN really give 16 million networks?

The 24-bit VNI space does, but practical limits arrive much sooner — the tunnel mesh, the number of ports per agent and the ML2 driver's database load all bite long before the VNI range is exhausted.

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