Prefix Expansion Calculator
List child prefixes at a selected length.
Prefix Expansion 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 Prefix Expansion Calculator Works?
This tool works by stepping through a parent prefix in increments of the child block size and listing every child prefix.
Enter the parent prefix
Type a block such as
10.20.0.0/22.Choose the child length
Pick a longer prefix, such as
/25.Step through the parent
The calculator starts at the parent network and adds 128 addresses (the /25 block size) per step.
List each child
Each row shows the child prefix, host range, and broadcast. A /22 into /25 gives 8 rows.
Prefix Expansion Calculator Formula
The formula is Child_k = Parent Network + k × 2^(32 − c), for k from 0 to 2^(c − p) − 1.
Kth child
C_k = N + k × 2^(32−c)c is the child prefix length.
Number of children
2^(c − p)Parent length p, child length c.
Last child
C_last = N + (2^(c−p) − 1) × 2^(32−c)Ends at the parent broadcast.
Worked example: 10.20.0.0/22 into /25
Children = 2^(25 − 22) = 8 Step = 2^7 = 128 addresses C_0 = 10.20.0.0/25 C_1 = 10.20.0.128/25 C_7 = 10.20.0.0 + 7 × 128 = 10.20.3.128/25
Prefix Expansion Calculator Conversion Table
The table expands 10.20.0.0/22 into its 8 child /25 prefixes, with the host range and broadcast of each child.
| # | Child Prefix | First Host | Last Host | Broadcast |
|---|---|---|---|---|
| 1 | 10.20.0.0/25 | 10.20.0.1 | 10.20.0.126 | 10.20.0.127 |
| 2 | 10.20.0.128/25 | 10.20.0.129 | 10.20.0.254 | 10.20.0.255 |
| 3 | 10.20.1.0/25 | 10.20.1.1 | 10.20.1.126 | 10.20.1.127 |
| 4 | 10.20.1.128/25 | 10.20.1.129 | 10.20.1.254 | 10.20.1.255 |
| 5 | 10.20.2.0/25 | 10.20.2.1 | 10.20.2.126 | 10.20.2.127 |
| 6 | 10.20.2.128/25 | 10.20.2.129 | 10.20.2.254 | 10.20.2.255 |
| 7 | 10.20.3.0/25 | 10.20.3.1 | 10.20.3.126 | 10.20.3.127 |
| 8 | 10.20.3.128/25 | 10.20.3.129 | 10.20.3.254 | 10.20.3.255 |
What is Prefix Expansion Calculator?
The Prefix Expansion Calculator is a tool that lists every child prefix of a chosen length inside a parent prefix.
Expansion turns a single block into a complete list. A /22 expanded to /24 gives 4 entries, and the same /22 expanded to /28 gives 64 entries. The list is the starting point for VLAN tables, route filters, and allocation sheets.
The Subnet Count Calculator returns only the number of children. The Prefix Expansion Calculator returns the children themselves, each with its host range and broadcast address.
What are the uses of Prefix Expansion Calculator?
There are 4 main uses of the Prefix Expansion Calculator, including allocation tables, prefix lists, and de-aggregation.
Building allocation tables
IPAM imports need one row per subnet. The expansion gives every row.
Writing prefix lists
BGP prefix lists sometimes list each more-specific prefix. The expansion produces the entries.
De-aggregating announcements
Traffic engineering announces more-specific prefixes. The expansion lists the /24s inside a /22.
Populating VLAN plans
Each child prefix maps to one VLAN in a numbered plan.