CIDR contains IP
Check IPv4 and IPv6 addresses against a CIDR range with a visual prefix comparison.
How it works & useful details
How it works & limits
Check up to 256 addresses, one per line, against an IPv4 or IPv6 CIDR. Network and final addresses are included. Host bits in the CIDR input are cleared and the normalized network is shown.
Addresses from a different family are outside the range. An IPv4-mapped IPv6 address remains IPv6 and is not silently converted to IPv4. Malformed entries stop the calculation. Input is limited to 64,000 characters.
Your working space
Explore the live view, then save copyable text and a downloadable file. Everything is calculated locally. Inputs are not stored; refreshing clears them.
These tools inspect address notation and ranges. They do not contact addresses, check availability or establish connectivity.
About this tool
CIDR contains IP checks whether an address falls inside a given network, for both IPv4 and IPv6, and shows the prefix comparison that produced the answer. It is the quick verification for access rules and allowlists.
Membership is computed by comparing the network bits locally, and the visual prefix view makes the decision easy to audit. No address is contacted, scanned, or resolved.
How to use it
- Enter the CIDR range and the address to test.
- Run the check to see whether the address is inside the range.
- Inspect the prefix comparison to understand the decision.
- Try another address against the same range.
Worked example
Checking 10.0.0.1, 10.255.255.255, and 192.168.1.1 against 10.0.0.0/8 matches the first two and rejects the third. A /8 fixes the first 8 bits, so every address from 10.0.0.0 to 10.255.255.255 is inside, including the first (network) and last (broadcast) addresses.
The check compares the leading prefix bits of each address with the network's. The visual comparison shows exactly where an address diverges, which is handy when a range boundary is not on an octet, such as 172.16.0.0/12.
Common questions it answers
- Is this client IP inside the office or VPN range before I add it to an allow-list?
- Does 172.31.255.10 fall in the private 172.16.0.0/12 block? (Yes: that block runs to 172.31.255.255.)
- Is an IPv6 address inside a /48 or /64 prefix assigned by a provider?
IPv4 and IPv6 are separate address families, so an IPv4 address never matches an IPv6 range. An IPv4-mapped IPv6 address such as ::ffff:10.0.0.1 is also kept as IPv6 rather than silently converted, because firewalls and access lists usually treat the two forms separately.
Frequently asked questions
Does it ping or scan the address?
No. Membership is a pure address calculation; no network traffic is generated.
Are IPv4-mapped IPv6 addresses handled?
Addresses are parsed in their native family. Convert mapped addresses explicitly if your rules mix families.
What is the limit?
Up to 256 addresses, one per line, within 64,000 characters of input. A malformed entry stops the calculation, so a typo cannot quietly appear as "not in range".