Free Cloud Subnet & CIDR Calculator
Most subnet calculators show total IP addresses only.
Cloud providers reserve additional IP addresses inside every subnet for platform services.
That means usable IP capacity is lower than traditional subnet math suggests.
This calculator shows how many IP addresses you can actually use in:
- Azure
- AWS
- Google Cloud (GCP)
- Generic (traditional networking)
That sounds like a small distinction, but it changes real deployment decisions. The difference between total addresses and usable addresses is exactly where a lot of cloud subnet plans go wrong.
What This Calculator Computes
Enter an IPv4 CIDR range (for example 10.0.0.0/24) and the tool calculates:
- Network address
- Broadcast address
- Subnet mask (dotted decimal)
- Total IP addresses (
2^(32 - prefix)) - Provider-reserved addresses
- Usable IP addresses
- First usable IP
- Last usable IP
This is most useful when you need an answer that is deployable rather than merely correct in textbook IPv4 math.
If you need classic subnet math without cloud-specific adjustments, use:
For a quick subnet mask and host-count lookup table, use:
Why Cloud Subnetting Behaves Differently
In traditional networking:
- Network address → reserved
- Broadcast address → reserved
In cloud environments, additional IPs are reserved for:
- Virtual routing infrastructure
- Default gateway behavior
- Platform networking services
- Internal system use
As a result:
A /27 does not provide 30 usable IPs in most cloud environments.
That gap is the reason teams get surprised by:
- private endpoint deployments that no longer fit
- node pools that cannot scale
- subnets that looked fine in a spreadsheet but fail under growth
- inconsistent planning between on-prem and cloud environments
Cloud Provider Reserved IP Models
Each cloud provider handles reserved IPs slightly differently.
Those differences are small enough to overlook and large enough to matter. On smaller subnets, losing four or five addresses can materially change whether a design still works.
Azure
- 5 IP addresses reserved per subnet
- First four and last IP are unusable
→ Learn more: Azure Reserved IP Addresses
AWS
- 5 IP addresses reserved per subnet
- First four and last IP are unusable
→ Learn more: AWS Reserved IP Addresses
Google Cloud (GCP)
- 4 IP addresses reserved per subnet
- First two and last two IPs are unusable
→ Learn more: GCP Reserved IP Addresses
Always validate workload-specific subnet requirements before deployment.
This calculator is intentionally focused on general subnet behavior. It is a planning aid, not a replacement for service-specific network guidance.
Generic (Classic) Subnet Mode
If you select Generic, the calculator follows traditional IPv4 rules:
/0–/30→ 2 reserved addresses (network + broadcast)/31→ 0 reserved (RFC 3021 point-to-point)/32→ single host
For detailed address breakdowns including wildcard masks, use:
Generic mode is useful when you are:
- validating classic network designs
- studying subnetting fundamentals
- comparing cloud behavior against textbook IPv4 rules
Example: /26 Subnet in Azure or AWS
- Total IP addresses: 64
- Reserved by provider: 5
- Usable IP addresses: 59
This difference becomes important when deploying:
- Kubernetes clusters
- Private endpoints
- Application gateways
- Internal load balancers
- Managed database services
Small subnets often exhaust faster than expected.
This is especially common when subnet space is shared by several platform features. A subnet that looks acceptable for virtual machines alone can become too small once you add managed services, Kubernetes nodes, private endpoints, or future growth.
Why This Calculation Matters Operationally
The question is rarely just “How many IPs does this subnet contain?”
The real question is usually one of these:
- Can this subnet survive the next scaling event?
- Do I have enough room for platform-reserved addresses and my workloads?
- If I choose this CIDR now, will I regret it after deployment?
Cloud networking punishes undersized subnets because expanding them later is often awkward, disruptive, or impossible without redesign.
Practical Subnet Sizing Guidance
When planning cloud environments:
/24→ Safe default when growth is expected/26→ Acceptable for moderate, predictable workloads/27or smaller → Only for tightly controlled deployments
Always leave headroom. Re-addressing a subnet after production deployment is operationally disruptive.
In practice:
/24is often the safest default when you are uncertain/26can work for tightly controlled environments/27and smaller should usually come with a specific reason, not just a desire to conserve address space
The goal is not to use the smallest valid subnet. The goal is to use the smallest subnet that still leaves room for reality.
How Many Usable IPs in Common Subnets? (Classic Mode)
| Prefix | Total IPs | Usable IPs |
|---|---|---|
| /30 | 4 | 2 |
| /29 | 8 | 6 |
| /28 | 16 | 14 |
| /27 | 32 | 30 |
| /26 | 64 | 62 |
| /25 | 128 | 126 |
| /24 | 256 | 254 |
| /23 | 512 | 510 |
| /22 | 1024 | 1022 |
Cloud providers reduce usable IPs further due to reserved addresses.
Use the calculator above whenever you need the provider-adjusted answer rather than the classic one.
Why does /27 not give 30 usable IPs in cloud?
Because Azure and AWS reserve 5 addresses, and GCP reserves 4.
For example:
/27(32 total)- Azure/AWS → 27 usable
- GCP → 28 usable
Use the calculator above to see exact values.
That difference becomes even more painful on small operational subnets where every address matters.
Does this validate service-specific subnet requirements?
No.
This calculator models general subnet behavior only.
Service-specific requirements (for example AKS, EKS, GKE, private endpoints, load balancer proxy subnets, etc.) must be validated separately.
If the subnet will be used by Kubernetes or cloud-managed services, continue with the more specialized planners and guides before locking in the design.
Related Networking Tools
IPv6 and advanced subnet planning tools are planned.
Designing Larger Cloud Networks?
When building hub-and-spoke or multi-VPC environments, correct subnet sizing early prevents painful redesign later.
Use this tool during architecture design — not after deployment.
That is the real purpose of a cloud usable IP calculator: catching bad assumptions while changing course is still easy.