Adjacent Range Checker
Identify directly neighboring ranges.
Adjacent Range 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.
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Subnet Boundary Checker
Check alignment to a specified prefix.
IP-in-Range Checker
Test whether an address belongs.
IP-in-Subnet Checker
Test membership within a CIDR block.
How Adjacent Range Checker Works?
This tool works by testing whether one range ends exactly 1 address before the other begins, then checking whether the joined range forms a single CIDR block.
Enter 2 ranges or prefixes
Type
10.4.2.0/24and10.4.3.0/24.Order the ranges
The checker places the lower range first.
Test End + 1 = Start
10.4.2.255 + 1 = 10.4.3.0→ adjacent.Check CIDR merge
The joined range
10.4.2.0 – 10.4.3.255equals10.4.2.0/23, so the pair merges into 1 prefix.
Adjacent Range Checker Formula
The formula is Adjacent = (End_A + 1 = Start_B), and CIDR-mergeable = Adjacent AND equal sizes AND Start_A aligned to 2 × size.
Adjacency
E_A + 1 = S_BNo gap and no overlap.
Equal size
|A| = |B|Needed for a CIDR merge.
Alignment
S_A mod (2 × |A|) = 0The lower block starts the larger block.
Merged prefix
p − 12 × /24 → /23.
Worked example: 10.4.1.0/24 and 10.4.2.0/24
10.4.1.255 + 1 = 10.4.2.0 → adjacent ✓ Equal sizes (256) ✓ 10.4.1.0 aligned to /23? 1 mod 2 = 1 → no ✗ Result: adjacent, but no single /23 covers both
Adjacent Range Checker Conversion Table
The table tests 8 pairs of CIDR blocks for adjacency and CIDR mergeability.
| Block A | Block B | Adjacent | Merges Into |
|---|---|---|---|
| 10.4.2.0/24 | 10.4.3.0/24 | Yes | 10.4.2.0/23 |
| 10.4.1.0/24 | 10.4.2.0/24 | Yes | No single prefix (2 blocks) |
| 192.168.0.0/25 | 192.168.0.128/25 | Yes | 192.168.0.0/24 |
| 192.168.0.128/25 | 192.168.1.0/25 | Yes | No single prefix (2 blocks) |
| 172.16.0.0/23 | 172.16.2.0/24 | Yes | No single prefix (2 blocks) |
| 10.0.0.0/24 | 10.0.2.0/24 | No | — |
| 203.0.113.64/26 | 203.0.113.128/26 | Yes | No single prefix (2 blocks) |
| 10.0.0.0/9 | 10.128.0.0/9 | Yes | 10.0.0.0/8 |
What is Adjacent Range Checker?
The Adjacent Range Checker is a tool that identifies whether 2 IPv4 ranges sit directly next to each other with no gap and no overlap.
Adjacency is the first condition for summarizing 2 blocks. Adjacent blocks of equal size merge into 1 shorter prefix only when the lower block starts on the boundary of the merged size. 10.4.2.0/24 and 10.4.3.0/24 merge into a /23, and 10.4.1.0/24 and 10.4.2.0/24 do not.
The checker reports both conditions, so the difference between "adjacent" and "summarizable" is clear.
What are the uses of Adjacent Range Checker?
There are 4 main uses of the Adjacent Range Checker, including summarization prep, allocation growth, and list merging.
Preparing route summaries
Adjacent, aligned pairs collapse into 1 route.
Growing allocations in place
A subnet grows in place only when the adjacent block is free and the pair is aligned.
Merging range lists
Adjacent ranges join into 1 entry in range-based systems.
Explaining failed summaries
The checker shows why 2 neighboring /24s do not form a /23.