Subnet Count Calculator
Count child prefixes within a parent.
Subnet Count 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 IPv4 Subnetting & RangeAll Tools
IPv4 Range Calculator
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Network Address Calculator
Find the subnet’s network address.
IPv4 Broadcast Address Calculator
Find broadcast addresses where applicable.
First and Last Address Calculator
Identify a range’s endpoints.
Usable IPv4 Host Range Calculator
Apply the selected subnet-use rules.
How Subnet Count Calculator Works?
This tool works by subtracting the parent prefix length from the child prefix length and raising 2 to the difference.
Enter the parent prefix
Type the parent block, for example
10.0.0.0/16.Enter the child prefix
Type the target size, such as
/24.Count the borrowed bits
Borrowed bits = 24 − 16 = 8.
Compute the count
2^8 = 256 child subnets, each with 254 usable hosts.
Subnet Count Calculator Formula
The formula is Subnets = 2^(child prefix − parent prefix).
Child subnets
S = 2^(c − p)Requires c ≥ p.
Borrowed bits
b = c − pBits moved from the host field to the network field.
Hosts per child
H = 2^(32 − c) − 2Standard rule.
Total usable hosts
S × HUsable hosts across all child subnets.
Worked example: /16 parent into /26 children
Borrowed bits = 26 − 16 = 10 Subnets = 2^10 = 1,024 Hosts each = 2^6 − 2 = 62 Total hosts = 1,024 × 62 = 63,488
Subnet Count Calculator Conversion Table
The table shows how many child subnets fit in a /16 parent at 13 child prefix lengths, with hosts per child and total hosts.
| Parent | Child Prefix | Borrowed Bits | Child Subnets | Hosts per Child | Total Hosts |
|---|---|---|---|---|---|
| /16 | /18 | 2 | 4 | 16,382 | 65,528 |
| /16 | /19 | 3 | 8 | 8,190 | 65,520 |
| /16 | /20 | 4 | 16 | 4,094 | 65,504 |
| /16 | /21 | 5 | 32 | 2,046 | 65,472 |
| /16 | /22 | 6 | 64 | 1,022 | 65,408 |
| /16 | /23 | 7 | 128 | 510 | 65,280 |
| /16 | /24 | 8 | 256 | 254 | 65,024 |
| /16 | /25 | 9 | 512 | 126 | 64,512 |
| /16 | /26 | 10 | 1,024 | 62 | 63,488 |
| /16 | /27 | 11 | 2,048 | 30 | 61,440 |
| /16 | /28 | 12 | 4,096 | 14 | 57,344 |
| /16 | /29 | 13 | 8,192 | 6 | 49,152 |
| /16 | /30 | 14 | 16,384 | 2 | 32,768 |
What is Subnet Count Calculator?
The Subnet Count Calculator is a tool that returns how many child subnets of one prefix length fit inside a parent prefix. A /16 holds 256 /24 subnets and 1,024 /26 subnets.
Each bit borrowed from the host field doubles the number of child subnets. The Subnet Count Calculator shows this trade-off: more subnets mean fewer hosts per subnet.
Classful-era textbooks subtracted 2 subnets for the "subnet zero" and "all-ones subnet". RFC 1878 and modern devices allow both, so the calculator uses the full 2^b count.
What are the uses of Subnet Count Calculator?
There are 4 main uses of the Subnet Count Calculator, including address plan sizing, VLAN capacity, and exam math.
Sizing address plans
A /16 carved into /24s supports 256 VLANs or sites.
Checking VLAN capacity
A campus with 300 VLANs needs more than a /16 split into /24s.
Comparing design options
Engineers compare 64 × /22 against 256 × /24 from the same /16.
Solving exam questions
Exams ask "how many subnets does borrowing 5 bits create". The answer is 2^5 = 32.