AWS VPC Subnet Capacity Calculator
Account for AWS address reservations.
AWS VPC Subnet Capacity 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
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VLSM Allocation Validator
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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 AWS VPC Subnet Capacity Calculator Works?
This tool works by subtracting the 5 addresses Amazon Web Services reserves in every subnet from the subnet size.
Enter the subnet CIDR
Type an AWS subnet, such as
10.0.16.0/20. AWS accepts IPv4 subnets from /16 to /28.Identify the 5 reserved addresses
Network address, VPC router (+1), DNS (+2), future use (+3), and broadcast (last).
Subtract the reservations
4,096 − 5 = 4,091 usable addresses.
Show the reserved list
The calculator prints each reserved address so it is never assigned by mistake.
AWS VPC Subnet Capacity Calculator Formula
The formula is Usable = 2^(32 − p) − 5, for prefixes /16 to /28.
Usable
2^(32−p) − 5AWS reservation rule.
Reserved +0, +1, +2, +3
Network, VPC router, DNS, future useThe first 4 addresses.
Reserved last
Broadcast positionAWS does not support broadcast but reserves it.
Valid sizes
/16 to /2865,536 down to 16 addresses.
Worked example: 10.0.16.0/20
Total = 4,096 Reserved = 10.0.16.0, 10.0.16.1, 10.0.16.2, 10.0.16.3, 10.0.31.255 Usable = 4,096 − 5 = 4,091 Range = 10.0.16.4 – 10.0.31.254
AWS VPC Subnet Capacity Calculator Conversion Table
The table lists usable AWS addresses for all 13 subnet sizes from /16 to /28, next to the standard usable count.
| Prefix | Total | Standard Usable (−2) | AWS Usable (−5) | First Usable Address (10.0.0.0 base) |
|---|---|---|---|---|
| /16 | 65,536 | 65,534 | 65,531 | 10.0.0.4 |
| /17 | 32,768 | 32,766 | 32,763 | 10.0.0.4 |
| /18 | 16,384 | 16,382 | 16,379 | 10.0.0.4 |
| /19 | 8,192 | 8,190 | 8,187 | 10.0.0.4 |
| /20 | 4,096 | 4,094 | 4,091 | 10.0.0.4 |
| /21 | 2,048 | 2,046 | 2,043 | 10.0.0.4 |
| /22 | 1,024 | 1,022 | 1,019 | 10.0.0.4 |
| /23 | 512 | 510 | 507 | 10.0.0.4 |
| /24 | 256 | 254 | 251 | 10.0.0.4 |
| /25 | 128 | 126 | 123 | 10.0.0.4 |
| /26 | 64 | 62 | 59 | 10.0.0.4 |
| /27 | 32 | 30 | 27 | 10.0.0.4 |
| /28 | 16 | 14 | 11 | 10.0.0.4 |
What is AWS VPC Subnet Capacity Calculator?
The AWS VPC Subnet Capacity Calculator is a tool that returns the number of IPv4 addresses available for resources in an Amazon VPC subnet, after AWS reserves 5 addresses.
AWS reserves the first 4 addresses and the last address of every subnet. In a /24, those are .0 (network), .1 (VPC router), .2 (Amazon DNS), .3 (future use), and .255 (broadcast). A /24 therefore holds 251 usable addresses, not 254.
The reservation hits small subnets hardest. A /28, the smallest AWS subnet, loses 5 of 16 addresses, which leaves 11.
What are the uses of AWS VPC Subnet Capacity Calculator?
There are 4 main uses of the AWS VPC Subnet Capacity Calculator, including EKS sizing, Lambda planning, and VPC design.
Sizing EKS subnets
Amazon EKS with the VPC CNI gives every pod a VPC address. Clusters need large subnets, often /19 or bigger.
Planning Lambda in a VPC
VPC-attached Lambda functions use ENIs from subnet capacity.
Designing multi-AZ VPCs
Each Availability Zone needs its own subnet with enough usable addresses.
Avoiding reserved-address errors
Assigning
.1to.3as static IPs fails in AWS.