Custom Cloud Subnet Calculator
Apply entered provider reservation rules.
Custom Cloud Subnet Calculator: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
Related Planning, VLSM & CloudAll Tools
VLSM Calculator
Allocate differently sized subnets.
VLSM Allocation Validator
Check alignment, capacity, and overlaps.
Supernet Calculator
Find a covering parent prefix.
Exact CIDR Aggregator
Merge prefixes without adding addresses.
Route Summary Calculator
Summarize networks and identify included gaps.
DHCP Scope Calculator
Plan allocation boundaries within a subnet.
How Custom Cloud Subnet Calculator Works?
This tool works by applying a user-entered reservation rule, a count of addresses reserved at the start and end of each subnet, to compute usable capacity for any cloud provider.
Enter the subnet
Type a CIDR such as
10.2.0.0/24.Enter the reservation rule
Type the count reserved at the start and the count reserved at the end. Azure uses 4 + 1, and Google Cloud uses 2 + 2.
Subtract the reservations
Usable = total − start reserve − end reserve.
Compare providers
The calculator shows the same subnet under AWS, Azure, Google Cloud, and the custom rule side by side.
Custom Cloud Subnet Calculator Formula
The formula is Usable = 2^(32 − p) − R_start − R_end, where R_start and R_end come from the provider rule.
Usable
2^(32−p) − R_s − R_eAny provider rule.
AWS rule
R_s = 4, R_e = 15 reserved.
Azure rule
R_s = 4, R_e = 15 reserved.
Google Cloud rule
R_s = 2, R_e = 24 reserved in a primary range.
Worked example: 10.2.0.0/24
AWS = 256 − 5 = 251 Azure = 256 − 5 = 251 Google Cloud = 256 − 4 = 252 Custom (3+1) = 256 − 4 = 252
Custom Cloud Subnet Calculator Conversion Table
The table compares usable addresses across AWS, Azure, Google Cloud, and a custom 3 + 2 rule for 10 subnet sizes.
| Prefix | Total | AWS (−5) | Azure (−5) | Google Cloud (−4) | Custom 3 + 2 (−5) |
|---|---|---|---|---|---|
| /16 | 65,536 | 65,531 | 65,531 | 65,532 | 65,531 |
| /18 | 16,384 | 16,379 | 16,379 | 16,380 | 16,379 |
| /20 | 4,096 | 4,091 | 4,091 | 4,092 | 4,091 |
| /22 | 1,024 | 1,019 | 1,019 | 1,020 | 1,019 |
| /23 | 512 | 507 | 507 | 508 | 507 |
| /24 | 256 | 251 | 251 | 252 | 251 |
| /25 | 128 | 123 | 123 | 124 | 123 |
| /26 | 64 | 59 | 59 | 60 | 59 |
| /27 | 32 | 27 | 27 | 28 | 27 |
| /28 | 16 | 11 | 11 | 12 | 11 |
Azure reserves the network address, .1 (gateway), .2 and .3 (Azure DNS), and the last address. Google Cloud reserves the network, gateway, second-to-last, and last addresses of a primary range.
What is Custom Cloud Subnet Calculator?
The Custom Cloud Subnet Calculator is a tool that computes usable subnet capacity under any provider's address reservation rule, entered as addresses reserved at the start and at the end of each subnet.
Every cloud platform reserves a few addresses per subnet for its own services, and the counts differ. AWS and Azure reserve 5, and Google Cloud reserves 4 in primary ranges. Private clouds, OpenStack deployments, and network appliances add their own rules.
A single rule input covers all of them, so multi-cloud designs use one consistent capacity table.
What are the uses of Custom Cloud Subnet Calculator?
There are 4 main uses of the Custom Cloud Subnet Calculator, including multi-cloud design, private cloud planning, and appliance subnets.
Designing multi-cloud networks
Equal workloads in AWS and Google Cloud need slightly different subnet sizes.
Planning private clouds
OpenStack and VMware NSX environments apply their own reservations.
Sizing appliance subnets
Firewall HA pairs and load balancers reserve extra addresses for VIPs.
Checking small subnets
Reservations matter most on /27 and /28 subnets, where every address counts.