Address Index Calculator
Find an address’s position within a range.
Address Index 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.
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First and Last Address Calculator
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Usable IPv4 Host Range Calculator
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How Address Index Calculator Works?
This tool works by subtracting the first address of a range from a target address. The difference is the index, and index 0 is the first address.
Enter the range
Type a CIDR block such as
10.8.0.0/21or a start–end range.Enter the target address
Type the address to locate, for example
10.8.2.50.Subtract the start
int(10.8.2.50) − int(10.8.0.0)= 2 × 256 + 50 = 562.Report position and share
The calculator returns index 562 and the position as a share of the range: 562 ÷ 2,047 ≈ 27.5 %.
Address Index Calculator Formula
The formula is Index = int(Address) − int(Start), valid while Start ≤ Address ≤ End.
Index
i = int(A) − int(Start)Zero-based position.
Index from the end
j = int(End) − int(A)Distance to the last address.
Relative position
Share = i ÷ (Size − 1) × 100 %0 % at the start, 100 % at the end.
Membership check
Start ≤ A ≤ EndAddresses outside the range have no index.
Worked example: 10.8.2.50 in 10.8.0.0/21
Start = 10.8.0.0 End = 10.8.7.255 Size = 2,048 Index = (2 × 256) + 50 = 562 From end = 2,047 − 562 = 1,485 Share = 562 ÷ 2,047 ≈ 27.45 %
Address Index Calculator Conversion Table
The table shows the index, index from the end, and relative position of 11 addresses inside 10.8.0.0/21.
| Address | Index | Index from End | Position |
|---|---|---|---|
| 10.8.0.0 | 0 | 2,047 | 0.00 % |
| 10.8.0.1 | 1 | 2,046 | 0.05 % |
| 10.8.0.255 | 255 | 1,792 | 12.46 % |
| 10.8.1.0 | 256 | 1,791 | 12.51 % |
| 10.8.2.50 | 562 | 1,485 | 27.45 % |
| 10.8.3.255 | 1,023 | 1,024 | 49.98 % |
| 10.8.4.0 | 1,024 | 1,023 | 50.02 % |
| 10.8.5.128 | 1,408 | 639 | 68.78 % |
| 10.8.6.200 | 1,736 | 311 | 84.81 % |
| 10.8.7.254 | 2,046 | 1 | 99.95 % |
| 10.8.7.255 | 2,047 | 0 | 100.00 % |
What is Address Index Calculator?
The Address Index Calculator is a tool that returns the numeric position of an IPv4 address inside a range or subnet. The first address has index 0.
The index is the reverse of the Nth address calculation. The Nth Address Calculator turns a position into an address, and the Address Index Calculator turns an address into a position.
Indexes are useful wherever addresses map to slots, array positions, or bitmap entries. IPAM systems store subnet usage as bitmaps, and the index of each address points to its bit.
What are the uses of Address Index Calculator?
There are 4 main uses of the Address Index Calculator, including bitmap allocation, template checks, and capacity tracking.
Mapping addresses to bitmap slots
IPAM tools store 1 bit per address. The index gives the bit number for each address.
Checking template compliance
A rule such as "printers use indexes 20 to 29" is checked by computing the index of each printer.
Tracking fill level
The highest assigned index shows how far into a subnet the allocations reach.
Converting addresses for scripts
Scripts that loop over subnets use indexes instead of dotted addresses.