OpenShift Cluster Network CIDR Planning Cheatsheet

2026-02-15 574 words 3 minutes

Contents

Reference tables for planning clusterNetwork sizing in OpenShift. The two key parameters are:

clusterNetwork.cidr — the overall pod IP space allocated across the cluster
clusterNetwork.hostPrefix — the subnet prefix length carved out per node from the cluster CIDR

How it works

Each node receives a dedicated subnet of size /hostPrefix from the clusterNetwork.cidr pool. The formulas are:

Value	Formula
Max nodes	`2^(hostPrefix − clusterPrefix)`
Pod IPs per node	`2^(32 − hostPrefix) − 2`
Total pod capacity	`Max nodes × Pod IPs per node`

Note

hostPrefix must always be greater than the cluster CIDR prefix (a larger number = smaller subnet). Values below /25 per node are unusual in production — Kubernetes itself consumes several IPs per node (kube-proxy, host-network pods, etc.).

Default OpenShift configuration

networking:
  clusterNetwork:
    - cidr: 10.128.0.0/14
      hostPrefix: 23
  serviceNetwork:
    - 172.30.0.0/16
  networkType: OVNKubernetes

This gives: 512 max nodes × 510 pods/node = ~261,120 total pod slots.

Combination tables

Cluster CIDR /14 (default — 262,144 total IPs)

hostPrefix	Max nodes	Pod IPs/node	Total pod slots	Typical use case
/23	512	510	261,120	OCP default — large clusters
/24	1,024	254	260,096	Many nodes, moderate pod density
/25	2,048	126	258,048	Very many nodes, low pod density
/26	4,096	62	254,000	Edge / IoT fleet, minimal pods/node

Cluster CIDR /16 (65,536 total IPs)

hostPrefix	Max nodes	Pod IPs/node	Total pod slots	Typical use case
/23	128	510	65,280	Medium cluster, high pod density
/24	256	254	65,024	Medium cluster, balanced
/25	512	126	64,512	Many small nodes
/26	1,024	62	63,488	Large node count, minimal pods

Cluster CIDR /18 (16,384 total IPs)

hostPrefix	Max nodes	Pod IPs/node	Total pod slots	Typical use case
/23	32	510	16,320	Small cluster, high pod density
/24	64	254	16,256	Small cluster, balanced
/25	128	126	16,128	Compact cluster, many nodes
/26	256	62	15,872	Edge cluster, many small nodes

Cluster CIDR /20 (4,096 total IPs)

hostPrefix	Max nodes	Pod IPs/node	Total pod slots	Typical use case
/23	8	510	4,080	Minimal cluster, dense pods
/24	16	254	4,064	Dev / lab cluster
/25	32	126	4,032	SNO + workers, low density
/26	64	62	3,968	Edge micro-cluster

Cluster CIDR /22 (1,024 total IPs)

hostPrefix	Max nodes	Pod IPs/node	Total pod slots	Typical use case
/24	4	254	1,016	SNO or 3-node compact
/25	8	126	1,008	Compact + a few workers
/26	16	62	992	Very small lab

Constraints and rules

Rule	Detail
`hostPrefix > clusterPrefix`	A node subnet must fit inside the cluster CIDR
`hostPrefix ≤ 30`	`/31` and `/32` leave no usable IPs
`hostPrefix ≥ 23` recommended	Fewer than 126 pods/node causes scheduling pressure
`clusterNetwork` ≠ `serviceNetwork`	The two ranges must not overlap
`clusterNetwork` ≠ machine/node CIDRs	Must not overlap with the machine network

Quick reference: pods per node by hostPrefix

hostPrefix	Subnet size	Usable pod IPs
/21	2,048	2,046
/22	1,024	1,022
/23	512	510
/24	256	254
/25	128	126
/26	64	62
/27	32	30
/28	16	14

install-config.yaml snippet

networking:
  networkType: OVNKubernetes
  clusterNetwork:
    - cidr: 10.128.0.0/14   # adjust to your sizing
      hostPrefix: 23          # adjust pods/node target
  serviceNetwork:
    - 172.30.0.0/16
  machineNetwork:
    - cidr: 192.168.0.0/24   # must not overlap with clusterNetwork or serviceNetwork

Warning

clusterNetwork and serviceNetwork cannot be changed after installation. Size conservatively — it is always safe to choose a larger CIDR than currently needed.

Verify on a running cluster

oc get network cluster -o jsonpath='{.spec.clusterNetwork}' | jq .

Expected output:

[
  {
    "cidr": "10.128.0.0/14",
    "hostPrefix": 23
  }
]