IP in CIDR check
Checks one IPv4 or IPv6 address against up to 100 CIDR blocks and returns, for each, whether it contains the address, plus the list of blocks that do. Blocks of the other IP version never match.
Updated 2026-10-27
What this tool checks
The address is compared with each CIDR block you give. A block with host bits set, such as 192.0.2.5/24, is read as its network, 192.0.2.0/24. The result has a row per block and the list of blocks that contain the address.
How to read the result
Use it to answer questions such as "is this sender inside my SPF ip4 terms?" or "is this client in the allow list?". To see what a single block covers, use the CIDR calculator.
Limitations
- Nothing is looked up: the tool compares numbers and does not know who uses the address.
Use it from code
API reference for IP in CIDR check
curl -i -X POST https://api.lookproof.dev/v1/ip/contains \
-H 'content-type: application/json' \
-d '{"cidrs": ["192.0.2.0/24", "198.51.100.0/24", "2001:db8::/32"], "ip": "192.0.2.10"}'A request without payment is answered with 402 Payment Required and the exact price. See the API reference.
Limits
| Input | IP address, Networks (CIDR) |
|---|---|
| Deadline | 2 seconds |
| Freshness | Computed from the input; nothing is looked up |
| Price | $0.002 per check |
Findings this tool can report
This tool reports no findings: the result is the computed data.
Frequently asked questions
What happens with blocks of the other IP version?
They never contain the address. An IPv4 address is not in an IPv6 block, and an IPv4-mapped IPv6 address such as ::ffff:192.0.2.1 is not in an IPv4 block. Mixing versions in one request is allowed.
What if the blocks overlap?
Every block is checked on its own, so an address can be in several of them. The list of matches names each network once.
How many blocks can I check?
Up to 100, in the browser and through the API. A longer list is refused rather than cut short.