Subnet Boundary Checker
Check alignment to a specified prefix.
Subnet Boundary Checker: 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 Subnet Math & Set OperationsAll Tools
Bitwise Subnet Calculator
Show address-and-mask operations.
IP Address Offset Calculator
Add or subtract address positions.
Address Distance Calculator
Count positions between two addresses.
IP-in-Range Checker
Test whether an address belongs.
IP-in-Subnet Checker
Test membership within a CIDR block.
Same Subnet Checker
Compare addresses under a selected mask.
How Subnet Boundary Checker Works?
This tool works by testing whether every host bit of an address is 0 for a given prefix length. An address that passes can start a subnet of that size.
Enter an address
Type an address such as
10.1.6.0.Enter a prefix
Type the prefix to check, such as
/23.Test the host bits
The checker computes
address AND wildcard. A result of 0 means the address is aligned.Suggest the nearest boundaries
An unaligned address returns the aligned boundary below and above it.
Subnet Boundary Checker Formula
The formula is Aligned = (int(IP) mod 2^(32 − p)) = 0, which equals (IP AND Wildcard) = 0.
Alignment test
int(IP) mod 2^(32−p) = 0Divisible by the block size.
Boundary below
floor(int(IP) ÷ 2^(32−p)) × 2^(32−p)Nearest lower boundary.
Boundary above
Boundary below + 2^(32−p)Nearest higher boundary.
Largest block
p_min = 32 − trailing zero bitsLargest prefix that can start at the address.
Worked example: 10.1.6.0 against /23 and /22
Octet 3 = 6 = 00000110 /23 → block 2 in octet 3 → 6 mod 2 = 0 → aligned ✓ /22 → block 4 in octet 3 → 6 mod 4 = 2 → not aligned ✗ /22 boundaries: 10.1.4.0 below, 10.1.8.0 above
Subnet Boundary Checker Conversion Table
The table checks 10 addresses for alignment at /22, /24, and /26, and lists the largest block each address can start.
| Address | /22 Aligned | /24 Aligned | /26 Aligned | Largest Block Starting Here |
|---|---|---|---|---|
| 10.1.0.0 | Yes | Yes | Yes | /16 |
| 10.1.4.0 | Yes | Yes | Yes | /22 |
| 10.1.6.0 | No | Yes | Yes | /23 |
| 10.1.6.64 | No | No | Yes | /26 |
| 10.1.6.128 | No | No | Yes | /25 |
| 10.1.6.32 | No | No | No | /27 |
| 10.1.7.0 | No | Yes | Yes | /24 |
| 10.1.8.0 | Yes | Yes | Yes | /21 |
| 10.1.6.1 | No | No | No | /32 |
| 10.0.0.0 | Yes | Yes | Yes | /7 |
What is Subnet Boundary Checker?
The Subnet Boundary Checker is a tool that confirms whether an IPv4 address sits on a valid starting boundary for a given prefix length.
A subnet of size 2^n must start at an address divisible by 2^n. 10.1.6.0 can start a /23 but not a /22. Routers reject or silently correct unaligned prefixes, which leads to routes that cover different addresses than intended.
The checker reports the largest block an address can start as a second result. 10.1.6.0 starts at most a /23, and 10.0.0.0 starts at most a /7.
What are the uses of Subnet Boundary Checker?
There are 4 main uses of the Subnet Boundary Checker, including allocation validation, supernet planning, and route checks.
Validating new allocations
A proposed
10.1.6.0/22fails the check before it reaches IPAM.Planning supernets
Two /24 blocks merge into a /23 only when the lower one sits on a /23 boundary.
Checking route entries
Static route networks must be aligned to their prefix.
Explaining exam answers
Wrong subnet answers often start on an unaligned address.