Usable Host Count Calculator
Account for applicable address reservations.
Usable Host 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
Find address boundaries from a prefix.
IPv4 Subnet Calculator
Calculate subnet details from address and mask.
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 Usable Host Count Calculator Works?
This tool works by taking the total address count of a prefix and subtracting the addresses reserved by the selected rule: 2 for standard subnets, 0 for /31 links, and 0 for /32 host routes.
Enter a prefix
Type a prefix such as
/26or a full block such as10.1.4.0/22.Compute the total
The calculator finds 2^(32 − p). A /26 gives 64.
Subtract reserved addresses
Standard subnets lose the network and broadcast addresses, leaving 62 for a /26.
Show the efficiency
The calculator divides usable by total. A /26 reaches 96.9 %, and a /30 reaches 50 %.
Usable Host Count Calculator Formula
The formula is Usable Hosts = 2^(32 − p) − 2 for prefixes /0 to /30.
Standard subnets
Usable = 2^(32−p) − 2Removes the network and broadcast addresses.
/31 links
Usable = 2RFC 3021 point-to-point rule.
/32 host
Usable = 1A single-address prefix.
Address efficiency
Efficiency = Usable ÷ Total × 100 %Larger subnets lose a smaller share to reservations.
Worked example: /21
Total = 2^(32−21) = 2,048 Reserved = 2 (network + broadcast) Usable = 2,048 − 2 = 2,046 hosts Efficiency = 2,046 ÷ 2,048 = 99.90 %
Usable Host Count Calculator Conversion Table
The table compares total addresses, reserved addresses, usable hosts, and efficiency for prefixes /16 to /32.
| Prefix | Total | Reserved | Usable Hosts | Efficiency |
|---|---|---|---|---|
| /16 | 65,536 | 2 | 65,534 | 100.00 % |
| /17 | 32,768 | 2 | 32,766 | 99.99 % |
| /18 | 16,384 | 2 | 16,382 | 99.99 % |
| /19 | 8,192 | 2 | 8,190 | 99.98 % |
| /20 | 4,096 | 2 | 4,094 | 99.95 % |
| /21 | 2,048 | 2 | 2,046 | 99.90 % |
| /22 | 1,024 | 2 | 1,022 | 99.80 % |
| /23 | 512 | 2 | 510 | 99.61 % |
| /24 | 256 | 2 | 254 | 99.22 % |
| /25 | 128 | 2 | 126 | 98.44 % |
| /26 | 64 | 2 | 62 | 96.88 % |
| /27 | 32 | 2 | 30 | 93.75 % |
| /28 | 16 | 2 | 14 | 87.50 % |
| /29 | 8 | 2 | 6 | 75.00 % |
| /30 | 4 | 2 | 2 | 50.00 % |
| /31 | 2 | 0 | 2 | 100.00 % |
| /32 | 1 | 0 | 1 | 100.00 % |
What is Usable Host Count Calculator?
The Usable Host Count Calculator is a tool that returns how many IPv4 addresses in a subnet can be assigned to devices after reserved addresses are removed. A /24 has 256 addresses and 254 usable hosts.
The count depends on the prefix length and on the reservation rule. Standard subnets reserve 2 addresses, RFC 3021 /31 links reserve 0, and a /32 identifies 1 host. Cloud platforms reserve more: Amazon Web Services (AWS) reserves 5 addresses per subnet, which the AWS VPC Subnet Capacity Calculator page covers.
The Usable Host Count Calculator returns the count only, so the number drops directly into capacity spreadsheets, procurement requests, and design reviews.
What are the uses of Usable Host Count Calculator?
There are 5 main uses of the Usable Host Count Calculator, including subnet sizing, waste measurement, and capacity planning.
Sizing subnets for device counts
A floor with 120 devices fits a /25 (126 usable hosts) and overflows a /26 (62 usable hosts).
Measuring address waste
Point-to-point links on /30 prefixes waste 50 % of each block. The efficiency column shows the cost.
Planning wireless networks
Guest Wi-Fi networks need large pools. A /22 gives 1,022 usable hosts for high-density venues.
Filling capacity reports
Capacity dashboards compare used addresses with usable hosts, not with total addresses.
Answering exam questions
Subnetting exams ask "how many hosts per subnet". The calculator returns the exact usable figure.