Create and Manage Subnets

Overview

Subnets define the IP address space within a network. Each subnet specifies a CIDR block, optional DHCP assignment, a gateway IP, and DNS resolvers that are pushed to instances at boot. A single network can host multiple subnets — e.g., one IPv4 and one IPv6 range — or separate subnets for different application tiers.

Prerequisites

An existing Polystack tenant network (see Create a Network)
Dashboard access or CLI configured with valid credentials

Add a Subnet to an Existing Network

Dashboard
CLI

Open the network

Navigate to Network > Networks and click the network name to open it.

Add a subnet

Click the Subnets tab, then click Create Subnet.

Configure CIDR and gateway

Field	Description
Subnet Name	Descriptive name, e.g., `db-subnet`
CIDR	CIDR block, e.g., `10.0.2.0/24`
IP Version	IPv4 or IPv6
Gateway IP	Optional — leave blank for auto-assign or enter `0.0.0.0` to disable

Configure DHCP and allocation options

Option	Description
Enable DHCP	Provide automatic IP assignment for instances on this subnet
Allocation Pools	Restrict the DHCP-assigned range, e.g., `10.0.2.10,10.0.2.200`
DNS	Comma-separated list of resolvers
Host Routes	Static routes injected via DHCP in `destination,nexthop` format

Use allocation pools to reserve address ranges for manually configured hosts such as database nodes or network appliances with static IPs.

Create the subnet

Click Create to save the subnet.

The subnet appears in the network’s Subnets tab with status Active.

Authenticate

Source your credentials file to authenticate with the Polystack platform:

Load credentials

source openrc.sh

Your administrator provides the RC (credentials) file for your project. See CLI Setup for configuration details.

Add subnet with DHCP

Create subnet with allocation pool

openstack subnet create db-subnet \
  --network app-network \
  --subnet-range 10.0.2.0/24 \
  --gateway 10.0.2.1 \
  --allocation-pool start=10.0.2.10,end=10.0.2.200 \
  --dns-nameserver 8.8.8.8

Add subnet without DHCP (static IPs)

Create subnet with DHCP disabled

openstack subnet create db-subnet-static \
  --network app-network \
  --subnet-range 10.0.2.0/24 \
  --no-dhcp

Use --no-dhcp for subnets where IP assignments are managed manually — e.g., database tiers where each node is pre-configured with a fixed address.

Verify

Show subnet details

openstack subnet show db-subnet

Confirm the CIDR, gateway, and DHCP settings match the intended configuration.

Subnet Management Operations

Update DNS Resolvers

Dashboard
CLI

Navigate to Network > Networks, open the subnet row, and click Edit Subnet. Update the DNS field in the Advanced Options.

Update DNS on a subnet

openstack subnet set app-subnet \
  --dns-nameserver 10.0.0.53 \
  --dns-nameserver 8.8.8.8

Existing instances pick up the new resolvers on their next DHCP renewal. To apply immediately inside the guest OS, run dhclient -r && dhclient on the instance.

Add Host Routes

Host routes are injected via DHCP and add static routing entries on the guest OS. Use them to direct traffic for specific CIDRs through a dedicated gateway.

Add host route to subnet

openstack subnet set app-subnet \
  --host-route destination=172.16.0.0/12,gateway=192.168.10.254

Disable or Enable DHCP

Disable DHCP on a subnet

openstack subnet set app-subnet --no-dhcp

Re-enable DHCP on a subnet

openstack subnet set app-subnet --dhcp

Disabling DHCP on a subnet that has instances running will cause those instances to fail IP renewal on the next lease expiry. Ensure all instances on the subnet are configured with static IPs before disabling DHCP.

Subnet Configuration Reference

Parameter	CLI Flag	Description
CIDR	`--subnet-range`	IP address range in CIDR notation
Gateway	`--gateway`	Default gateway IP for DHCP clients
DHCP	`--dhcp` / `--no-dhcp`	Enable or disable automatic IP assignment
Allocation Pool	`--allocation-pool`	Start/end range for DHCP-assigned addresses
DNS Servers	`--dns-nameserver`	Resolvers pushed to instances (repeat for multiple)
Host Routes	`--host-route`	Static routes injected via DHCP
IP Version	`--ip-version`	`4` for IPv4, `6` for IPv6

Next Steps

Routers and Gateways

Attach subnets to a router to enable inter-subnet routing and internet access

DNS Configuration

Configure DNS name servers and hostname resolution for your subnets

Create a Network

Create the network that hosts your subnets

Network Troubleshooting

Diagnose and resolve subnet connectivity and DHCP issues

Core Services

Other Services

Overview

Add a Subnet to an Existing Network

Open the network

Add a subnet

Configure CIDR and gateway

Configure DHCP and allocation options

Create the subnet

Authenticate

Add subnet with DHCP

Add subnet without DHCP (static IPs)

Verify

Subnet Management Operations

Update DNS Resolvers

Add Host Routes

Disable or Enable DHCP

Subnet Configuration Reference

Next Steps

Routers and Gateways

DNS Configuration

Create a Network

Network Troubleshooting

​Overview

​Add a Subnet to an Existing Network

Open the network

Add a subnet

Configure CIDR and gateway

Configure DHCP and allocation options

Create the subnet

Authenticate

Add subnet with DHCP

Add subnet without DHCP (static IPs)

Verify

​Subnet Management Operations

​Update DNS Resolvers

​Add Host Routes

​Disable or Enable DHCP

​Subnet Configuration Reference

​Next Steps

Routers and Gateways

DNS Configuration

Create a Network

Network Troubleshooting

Overview

Add a Subnet to an Existing Network

Subnet Management Operations

Update DNS Resolvers

Add Host Routes

Disable or Enable DHCP

Subnet Configuration Reference

Next Steps