Calculate IPv4 and IPv6 subnets, usable host ranges, and prefix lengths — client-side, no data sent to servers.
Plan VPC CIDRs, firewall rules, Docker networks, and ACL wildcards with exact bit math.
Free CIDR & Subnet Calculator exists to calculate an IPv4 or IPv6 network from an address and a prefix length. It is not a second copy of the tool homepage. The homepage introduces the whole product. This page stays on one job so a search for that job lands on instructions you can follow without hunting through other tabs. Read the result on this page against the input you actually used. A screenshot without the input is not evidence. If the result surprises you, change one thing and run it again before you change your VPN, browser, or server config.
Work through Free CIDR & Subnet Calculator in order. 1. Choose IPv4 or IPv6 to match the address you have. 2. Enter the address and the prefix. 3. Read network, broadcast or last address, and the usable range. 4. Use VLSM only when you are splitting, not when you are identifying one network. Write down the input and the output together. When you ask someone for help, send both. Repeat the same input once. A stable tool returns the same answer. If it does not, the input changed or the page is talking to a different network path than you think.
The calculator does not configure routers. A prefix of /32 or /128 is a single host. Overlapping ranges are your addressing plan's problem to notice. Treat the output as a measurement, then decide. RookVPN does not log the contents of a client-side tool, and a measurement is not a promise that every other app on the device behaves the same way. Compare a second path when the decision matters: a terminal command, another browser, or the matching guide linked below.
Example: 192.168.10.40/24 is the network 192.168.10.0 with usable hosts .1 through .254 and broadcast .255. If you needed a /28 inside that, go to the VLSM page rather than guessing the third octet. The calculator will not refuse a plan that overlaps another VLAN. You compare the ranges yourself.
After you finish Free CIDR & Subnet Calculator, open the IPv4, IPv6, or VLSM page if the next question is different from the one this page answers. Stay here if you are still on the same job. Extra pages help only when they answer a new question, such as a different algorithm, a different leak channel, or a different file type. The documentation link on this page is the long form of the same workflow, including the checks that do not fit in the tool UI.
Use Free CIDR & Subnet Calculator for one job: to calculate an IPv4 or IPv6 network from an address and a prefix length. Bring the before-and-after evidence with you if you change a setting. A VPN icon, a decoded token, a generated password, or a stripped file is not finished until you have checked the output the way this page describes. Then follow the related guide for the long version, and the sibling tool only when the next job is actually different. Keep secrets out of the same message as the link or the file that needs them. If a second run does not match the first, stop and find what changed instead of publishing the first result.
Example: 192.168.10.40/24 is the network 192.168.10.0 with usable hosts .1 through .254 and broadcast .255. If you needed a /28 inside that, go to the VLSM page rather than guessing the third octet. The calculator will not refuse a plan that overlaps another VLAN. You compare the ranges yourself. Work the page in this order: Choose IPv4 or IPv6 to match the address you have. Enter the address and the prefix. Read network, broadcast or last address, and the usable range. Use VLSM only when you are splitting, not when you are identifying one network. Limits that still apply after a clean result: The calculator does not configure routers. A prefix of /32 or /128 is a single host. Overlapping ranges are your addressing plan's problem to notice.
Yes at rookvpn.com/subnet.
The number of leading bits that identify the network.
On IPv4, the last address of the subnet, which is not a host.
No. It explains addresses. The leak test is a different tool.
Yes. Use the family that matches the address.
One IPv4 host.